What patents are there currently established for question and answer sites?

The Q&A site concept, insofar as the way you will be developing it, does not seem patentable. A lot of very different websites, some competing with each other, have the upvote-downvote feature.

In the world of software, you're more susceptible to copyright infringement than patent infringement.

Be wary, however, of SURPRISE!!! patent lawsuits as has been the case here (against Reddit, Digg, Fark, Slashdot & TechCrunch): http://www.techdirt.com/articles/20110114/22252012690/still-trying-to-track-down-who-controls-patent-used-against-reddit-digg-fark-slashdot-techcrunch.shtml#comments

This is not legal advice. Consult a patent attorney.

Don't worry about patents for now. Until you start making lots of money, no one is going to waste their time suing you for patent infringement. You can't get blood from a stone. Once you start making real money, then start worrying about it.

Do you infringe someone's patent? Almost certainly. There are probably 10,000 patents covering the basic functionality of Q&A sites. Determining whether you infringe any one patent, however, is a time-consuming and expensive process; to survey the field is much more expensive. Doing a search is like a needle in a haystack and not worth it.

One exception is patents that are being actively litigated, like the ones mentioned in another answer. Since this patent owner will likely go after others, it might be worth the effort to look at these patents avoid infringing them. Even with these, however, the patent owner probably has a lot of bigger fish to go after before going after you (unless the size of your business is comparable to Digg).

imho patents are most useful as a deterrent to keep other people out of the space, however if you don't have the money to be right in court then its not very useful.

1. If there is a patent that doesnt mean that all of its claims are valid, the validity of claims can only be determined in an absolute sense by going to court, the claims start out very broad.
2. For a patent to be valid it must be new, non obvious and useful.

you should do a search yourself if your really concerned for a few thousand dollars you can get a patent search done but that doesn't guarantee it will find everything. If you find something that concerns you see a IP lawyer and discuss it with them, the first startup I did was for a robot to prune grape vines and there where patents on this! when I took it to a IP lawyer he assured me there claims where not valid and that the patent had been written by an individual not a patent attorney.