I just noticed that MathOverflow is soliciting donations at this link, or click here to donate directly. If you haven't donated yet, please consider it -- the site is an important place for discussion, and I know that I benefitted from it quite a bit in the first couple years of grad school. From the post on Meta.MathOverflow:
“Long time followers of MathOverflow will know that as well as the website, hosted by StackExchange, and moderated by the moderation team with all of your help, there is an independent 501(c)(3) organization called MathOverflow, whose primary role is to ensure the long term success of the website. In particular, “MathOverflow Inc” has a legal agreement with StackExchange that allows us to separate amicably if the need arises, in particular taking the complete data and name, and allowing us to find our own hosting arrangements. While we certainly hope this won’t ever need to happen, and we’re very happy with all the help and support StackExchange provides us, it’s important that MathOverflow in this sense “belongs to the mathematicians”, held in trust by this legal body.
The board members of “MathOverflow Inc” are Anton Geraschenko, Ben Webster, David Zureick-Brown, François Dorais, Scott Morrison, Todd Trimble, Scott Carnahan, Mariano Suárez-Alvarez and Ravi Vakil. We meet (virtually) every year, and discuss long term issues around MathOverflow. In particular, projects such as obtaining a grant from the Alfred P. Sloan Foundation, generating trackbacks to the arXiv, and implementing the “citation search” feature on MathOverflow, supported by a Zentralblatt backend, have been overseen by the board. We’ve also done a small amount of work ensuring we know approximately what we’d do in the event of needing to leave StackExchange.
There are some minor expenses involved in making all this happen. There are the legal costs of starting and maintaining 501(c)(3) status, there have been some hosting costs for generating trackbacks and supporting the citation search feature (although due to Zentralblatt’s support these are now fairly minimal) and of course in order to be able to do those things we need a MathOverflow bank account, etc. During the development of the citation search feature we paid a MathOverflow user for some coding work for a demo.
So far, this has all been funded by a grant from the Alfred P. Sloan Foundation, and kind donations from a small number of generous donors (in fact, from amongst the board members). It’s time, however, to ask for a little more help, and this post serves to announce the existence of a link allowing you to donate to MathOverflow! We’re not looking for much at this point — really just enough to ensure we have the minor legal and hosting costs covered, and to reach the minimum balance on the bank account to avoid fees!”
If you prefer not to donate via PayPal, the post discusses other options.