Now that the million-or-so people who filled the Mall in Washington D.C. on Tuesday have made their way back home, Barack Obama is settling down in the Oval Office, ready to get to work. At the top of the new president's to-do list? Fixing the ailing economy.

During his campaign, Obama praised small businesses as a prime source of job creation. Following a long campaign tradition, he made a point of stopping in mom 'n' pop diners, small-town drugstores and other spots that highlighted the American entrepreneurial spirit.

But now it's crunch time. What impact will the new administration have on small businesses? Look at the new president's goals and priorities, and you'll see that there are three sectors where both big and small businesses might come out ahead.

1. Construction:

One of the first things the new president hopes to sign into law is a massive stimulus plan. Part would be spent on tax cuts, including an extension of business-tax cuts first offered last year as part of Bush's stimulus plan. But millions more would be earmarked for infrastructure projects, from rehabbing inner-city schools to modernizing past-their-prime bridges and highways.

That's good news for construction-equipment manufacturers like

Caterpillar

(CAT) - Get Report

. Until recently, the company had been weathering the tough economy by focusing on sales in booming overseas markets such as India and China. But sagging demand worldwide eventually dragged those markets down, too. In late December, Caterpillar announced salary cuts, a hiring freeze and layoffs at an Illinois engine-manufacturing plant.

Huge infrastructure spending in the U.S. could be a boon to Caterpillar. But it might make an even bigger difference to thousands of small construction-related businesses.

Electricians, plumbers and contractors earned ample overtime during the housing boom of the early 2000s, but now many are hustling to find work. While overall unemployment hovers around 7%, it's about twice that in the construction industry. According to the Bureau of Labor Statistics, about 800,000 construction jobs were lost in the past two years.

The Associated General Contractors of America, a trade group, is sponsoring a petition urging Congress to pass a stimulus package, making the case it's a win-win: "Investing in infrastructure will put men and women back to work now while giving our country a decades-long advantage over our global competitors."

Should the stimulus plan pass Congress, small construction businesses could get a piece of that government windfall.

2. Alternative energy:

Another important component of Obama's planned stimulus package is a focus on sustainable energy. Investing in new technologies will not only reduce our dependence on foreign oil and lower carbon emissions, it will create new jobs for companies innovative enough to fill this niche.

Major corporations are already leaping aboard the green bandwagon. Companies including

General Electric

(GE) - Get Report

,

Duke Energy

(DUK) - Get Report

,

Alcoa

(AA) - Get Report

and

ConocoPhillips

(COP) - Get Report

have banded together with environmental groups to form the U.S. Climate Action Partnership. Its goal? To urge Congress to pass climate-change legislation that's both environmentally effective and economically sustainable.

Of course, these companies aren't just acting out of nobility. They want to get in on the ground floor of a potentially lucrative new industry.

Which is exactly why small businesses can benefit, too. Entrepreneurs who focus on green energy can move more quickly and take bigger risks than their larger competitors. And if the Obama administration hands out tax breaks to innovative companies, it might fuel a landslide of creative new ideas.

3. Communications:

As the first BlackBerry-wielding tech-savvy president, Obama has talked about the country's "digital divide." High-speed Internet access is a key marketing, communication and sales tool for modern businesses, but many rural and inner-city communities without broadband access have been left behind.

If Obama succeeds in pushing through a plan to wire vast stretches of the country, that's great news for cable giants like

Comcast

(CMCSA) - Get Report

, but also for the many subcontractors who will be called on to do the actual digging and installing. Once millions of new users are online, they'll need troubleshooters and repair techs to keep things running smoothly. It's a perfect opportunity for small companies to shine.

It's far too early to say if the Obama era will be good for business overall. But small businesses that cater to the new president's top priorities should find reason to celebrate.

Elizabeth Blackwell is a freelance writer based in Chicago. She is the author of Frommer's Chicago guidebook, and writes for the Wall Street Journal, Chicago, and other national magazines.