Freelancing has its upsides: flexible hours, working from wherever, calling the shots, and generally doing it for yourself. But there’s one major downside that almost no freelancer has escaped: the deadbeat client who refuses to pay an invoice.

That abhorrent phenomenon is the driver behind the World’s Longest Invoice, a running tally from stiffed freelancers to non-paying clients. The effort was organized by the Freelancers Union, and the bill currently sits at $8.48 million as of this writing.

What’s on the invoice? Michel B. was stiffed out of $21,000 worth of software development; Cullen M. lost $60 in PHP work; Kate C. did $450 worth of web design that she never got paid for. On the invoice, you’ll see the occasional complaint about unpaid-for T-shirt design or wedding invitations, but by and large, the legions of unpaid are complaining about a lot of “pro bono” web work.

“In 2010 alone, more than 40 percent were stiffed,” the union’s Jaclyn Kessel said in a conversation with VentureBeat today. “77 percent of freelancers are stiffed at some point in their career.”

Deadbeat clients are possibly the single biggest pain of freelancers’ work. In spite of contractual obligations and even legal threats, these kinds of clients will steadfastly refuse to honor previous agreements or even acknowledge your repeated requests for payment. They might string you along for months with promises to pay, only to leave you hanging.

Sure, you could take them to small claims court, but even after the hours of bureaucracy involved in that process, you still might end up empty-handed.

“Right now, freelancers have to sue, or walk away,” said Kessel. “They don’t have the same Department of Labor Protections that traditional employees enjoy — that’s why we sponsored first of its kind legislation in New York State giving freelancers equal protection from deadbeat companies.”

So, the World’s Longest Invoice is a campaign to bring awareness to the issue. Basically, it’s an online counter where freelancers can add the amount they are still owed by deadbeat clients.

Strangely enough, these clients aren’t all no-name mom-and-pop shops or shady SMBs. “We’ve seen blue chip companies like Time demand a 5 percent ‘on-time’ fee, and we’ve seen small boutique firms close shop and open under different names just to avoid paying freelancers what they’re owed,” said Kessel.

The Freelancers Union, in addition to providing its members with healthcare options and other benefits, also advocates for legislation to make it harder for non-paying clients to get away with their unbelievably rude and actually criminal behavior. The union estimates that last year, freelancers lost around $4,600 each due to non-paying clients.

“We want to demonstrate the scope and the toll of the problem on new workers and our economy, so we can build a new set protections that makes sense for 21st century workers,” Kessel said. “In New York, we sponsored first-of-its-kind legislation giving freelancers equal Department of Labor protection from deadbeats — we hope it’ll serve as a national model.”