BOSTON (TheStreet) -- When a massive earthquake rocked Haiti this week, leveling the capital city of Port-au-Prince and leaving tens of thousands dead, relief agencies acted quickly.

So did the con men.

A warning released by the Federal Bureau of Investigation on Tuesday urges the well-intentioned "to apply a critical eye" before responding to requests for relief-related donations. A similar warning from the Better Business Bureau's Wise Giving Alliance compares the likely emergence of Haitian relief scams to similar e-mail schemes that emerged following the deadly tsunami of 2004 and Hurricane Katrina in 2005.

Man-made disasters offered more opportunities for Internet fraudsters than Mother Nature last year.

"Some scams were distinct in 2009 because of the economic climate and scammers' penchant for taking advantage of the top headlines," says Steve Cox, chief executive officer of the Council of Better Business Bureaus. "In places particularly hit by the housing crisis, bogus offers for foreclosure rescue or debt assistance ran rampant."

As one spamming strategy ceases to be effective, new tricks of the trade emerge to take its place. Nigerian scammers have been replaced by acai berry claims and fake auctions. Facebook fraud has supplemented phony lotteries and Ponzi schemes.

The Internet Crime Complaint Center, a joint partnership between the FBI and the National White Collar Crime Center, received 275,284 complaints in 2008, the most recent year for which complete statistics are available. The total dollar loss from these reported cases was $264.6 million.

Earlier this month, the Wise Giving Alliance released a list of its top reported rip-offs in 2009. Scurrilous "free-trial" offers were especially widespread last year.

Ads offering samples of tooth whiteners and acai berry-based supplements were especially ubiquitous, popping up on mainstream Web sites. Aside from the dubious claims about such products, anyone who requests a free sample usually finds they've been duped into paying more than $80 a month for recurring shipments.

In December, Visa ( V) blocked billing transactions for nearly 100 businesses involved in this practice.

The Wise Giving Alliance is also warning consumers about a new twist on the age-old con of "work-at-home" offers.

A number of sites claim to offer free learning materials on how to make money from home by using Google ( GOOG) or Twitter. Those who responded were suckered into a slew of ill-defined service charges and recurring bills.

Hucksters also reeled in their victims with offers of foreclosure prevention and debt relief programs. Despite promises of being able to help customers with their financial woes, all they did was add to them by charging hundreds of dollars upfront for assistance that would never be sent. Craigslist and eBay ( EBAY) are both popular destinations for tech-savvy grafters who either fence stolen goods or are trying to dig up a customer's personal information.

A popular variation, according to the Wise Giving Alliance, are "overpayment scams" that target small-business owners, landlords and sellers. The scammer sends a check for more than the amount requested, and then asks the victim to deposit the check and wire the "accidental" overage back to them. Ultimately, the check proves to be fake and the customer is never heard from again.

The use of "scareware" continued in 2009 with pop-ups disguised as a real-time anti-virus scan of the user's hard drive triggered by a compromised site. Once the "scan" appears, it can't be easily closed because clicking on any part of it will only download trojans and keyloggers. The only safe option is to quit the browser, shut the computer down and run a real anti-virus scan after restarting. According to the FBI, scareware is estimated to have caused more than $150 million in damage.

The increased popularity of social networking sites has led to a variety of schemes that target them. Typically, the goal is to steal user information and "hijack" profiles so that it can be used to promulgate spam that appears to come from a friend. Infected users are often unknowingly spreading malware by having infected sites posted on their page without their knowledge.

-- Reported by Joe Mont in Boston.