Replying to Dick Palais, any edit to wikipedia no matter now small is good, even if its just correcting a typo. One thing I've done is make a small edit to every page I visit, there is always something to be fixed. Once you've done a number of small edits you can do something more substantial. The mathematics reference desk is quite similar to MathOverflow and could be a good place to get started http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Mathematics
Replying to Marc Palm, it is quite easy to see who make edits to a page, just click on the history tab. A users contributions can be seen by going to a users page and selecting user contributions on the left.