There is also Science Exchange which is aiming to be a sort of E-lance/service marketplace for science in general. You can register there as a service provider. I have done this but so far I haven't been contacted about anything in terms of Bioinformatics and I'm not sure how active it is in terms of people posting needs. A fiar amount of providers though.
How important is compensation to you (relative to education and experience)?
If the answer is very important, then as Jeremey suggests, you're probably going to have a hard time finding short-term work. As is probably true with any highly-skilled area, it takes a substantial investment to get a new person up and running (and productive). So unless there is an exact match between what you're good at and what someone needs, it's tough for an employer to make that investment.
If the answer is not important (i.e., emphasis on gaining education and experience), then I think it would be exceptionally easy to find short-term projects. The investment by the "employer" is much lower so you're much more likely to find someone to serve as a mentor (that might later translate to a paying gig or a nice letter of reference). To find this type of project, I have two suggestions.
First, find an open source project that you're interested in, then lurk on the mailing list or check out the bug tracker. You could even use the Google Summer of Code as a starting point, since mentoring organizations have to propose projects suitable for a summer internship.
Second, identify a lab who does cool stuff and offer your services. Most PIs have too many ideas than hands to explore, and would be happy to pursue the possibility of free labor in exchange for some email mentorship. (My lab, for one, has happily offered project opportunities in exchange for authorship in the past. If you're interested in what we do, feel free to get in touch. </shameless_plug>)
Genohub.com has just started matching researchers who only need data analysis services with bioinformaticians. If you're a bioinformatician freelancer and would like to participate in our new service, describe what you do here: https://genohub.com/bioinformatics-service-listing/
Genohub connects service providers with researchers interested in sequencing and library preparation. Genohub takes service information from providers and project specifications from researchers and creates optimal sequencing packages while facilitating the entire order process, from shopping to data delivery.
Check us out, and let us know if you have any questions: firstname.lastname@example.org