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You know the old saw - "Big things appear in small packages"

So it goes with your investment portfolio, which these days can grow to large sums all through the use of a mobile device no more than six inches in size. That's right - you can run your investments in real time on your phone, through a selection of great mobile investment apps, more or less like a professional Wall Street trader.

That's a good deal, and in more ways than one.

It used to be that getting professional help with your stock, bond and fund investments came at a price - often up to 3% of your total portfolio assets, depending on the brokerage firm you were using.

Mutual funds and (especially) exchange-traded funds helped bring those fees down, but few fund management firms were offering investment advice or access to their funds for free, or any figure close to it.

That was then and this is now. In the digital age, there are plenty of low-cost or no-cost mobile apps that help you invest your money in the markets in myriad, effective ways. Which mobile investment apps stand out in a now-crowded digital investment age field?

These seven investment management apps should set the tone, if not save the day for investors looking for solid market advice, and at a minimal (or no) price.

1. Stash

This mobile investment app bills itself as the digital landing spot for investors looking for the best financial market tools - and it may be on to something. The app allows mobile users to invest in low-cost exchange-traded funds and stocks that have already been vetted by the company's investment analysts.

A quick review of the site reveals in-depth dives into major Fortune 500 stocks, like Abbott Laboratories (ABT - Get Report) , 3M (MMM - Get Report) , Caterpillar (CAT - Get Report) , and many, many others. The app links to major stock market analysis platforms like MarketWatch and Yahoo Finance then allows you to invest as little as $5.

As Stash puts it, for just $1 a month, you'll get a personal investment account with unlimited trades, education, and fractional investing for as little as $5. Account balances at $5,000 per year and over cost 0.25% annually.

2. Vault

The Vault app caters primarily to the retirement investment crowd, enabling any long-term investor to open up an individual retirement account (IRA), Roth IRA, or SEP IRA account (for self-employed career professionals.)

Not only does Vault provide invaluable advice on setting up and managing a retirement plan, it also allows users to steer a fixed percentage of their income into their retirement plan of choice.

That's especially useful if your company doesn't deduct your retirement savings from your paycheck, or if you work as a self-employed individual, and have no one to regularly deduct income and stash it away for your retirement fund.

The app is simple to use, comes with qualified and experienced investment advice from trained customer service representatives, and its low minimum cash amount ($1) and low monthly fee ($1) is a comfort for inexperienced retirement investors looking for a toe-hold to get started.

3. Personal Capital

The Personal Capital app is more of a birds-eye view of your investment portfolio.

The app updates and tracks all of your investment portfolio holdings, and evaluates your portfolio performance and your asset allocation and risk management needs. You can even swipe to check out particular gainers and losers in your investment portfolio, giving you a real-time glimpse of how you're doing in the markets, along with a comparison too that pegs your portfolio performance against major market indices, like the S&P 500 Index or the Russell 2000 Index.

Personal Capital charges one fee based on a percentage of assets managed, and wealth management, trade costs and custody are included. App users pay no trade commissions and the asset fee price starts at 0.89% of your first million dollars invested.

4. Stockpile

This app follows a burgeoning trend among mobile investment apps - taking an incremental amount of your money and investing it in the stock market.

Stockpile offers users the ability to buy fractional shares of hundreds of stocks and exchange-traded funds for as low as $5 per trade. The apps do charge a commission - at 99 cents per trade (compare that to E-Trade (ETFC - Get Report) , which charges $6.95 per trade), but there are no monthly fees or account minimums.

There's a great deal of buzz surrounding Stockpile as an app that can introduce the younger set to investing - and people who tout that approach aren't wrong. Think of a mom or dad who wants to introduce their 10-year-old to the investment world, at an affordable $5 a pop.

The app also has a gift card option where you can give a $20 (or more) gift card for junior's birthday, instead of buying a video game or a gift card to a big box retailer. That's a good deal, and it's a good launching point for a child's entrance into the money management world - a journey that can't start soon enough.

5. Wealthfront

Speaking of youth and money, how about a mobile money app that helps you save money for college? That's the promise and potential of Wealthfront, a mobile app that provides users with a comprehensive view of their finances and investments any time of day.

The app is designed with capital appreciation in mind, especially for college savings. It comes with a small army of smart, savvy finance and investment analysts who steer Wealthfront users to the investment strategy that fits them best, based on their investment risk profile and financial goals.

The app offers up 11 different ETFs to invest money in, and once you put cash into a fund, Wealthfront uses a robotics-based portfolio management technology to allocate funds as needed, and recommend specific market strategies like tax loss harvesting and portfolio diversification.

The app offers a College 529 investment plan option, as well, and is free of charge for the first $10,000 in assets under management on the Wealthfront platform (after $10,000 in assets, the company charges 0.25% of total assets, and requires a $500 minimum investment.

6. E-Trade

E-Trade broke the mold for low-cost investment trading fee models, but these days the company's $6.95 per-trade fee seems almost archaic - but don't judge E-Trade's mobile app on trading price alone.

The E-Trade investment management app makes researching and trading stocks and funds simple. Users can trade stocks, options, ETFs and mutual funds via the app. They also get streaming quotes, charts and portfolio data in real time, along with high-level help from E-Trade investment specialists in building a professional investment portfolio.

The app's live, built-in Bloomberg video feature keeps you up-to-date on current market conditions and analysis, and you can chat with an E-Trade customer service professional as you trade and build your portfolio.

There is a $500 account minimum, but you do get 500 commission-free trades if you deposit $10,000 into your E-Trade account.

7. Robinhood

Like Stockpile, Robinhood allows investors to get a piece of a good publicly-traded company in small bites, and in a commission-free manner - which is especially appealing to younger investors.

The app enables users to invest in stocks, ETFs, options and even cryptocurrencies with no fee (the company primarily earns its money on earned interest on un-invested money in customer accounts, selling customer trade order flows to brokers, and from selling ads) and no commissions. However, the app doesn't allow trading for bonds or mutual funds, which limits customer trading options.

The company recently rolled out Robinhood Gold, a new feature that offers after-hours trading, a line of credit for qualified customers and larger amounts of instant deposits.

You don't get much help in the form of investment education, research and customer support, but 5 million Robinhood users don't seem to mind - they're mostly there for the free trades.