The Brony UK Convention (B.U.C.K., but don't ask me how that works) is, fittingly enough, a convention to be held in Manchester during the 18th and 19th of August. Among the many activities planned is a charity auction which will sell off pony swag like plushies, costumes, customs, and art to interested bidders and dump all of the proceeds raised to a charity to be decided by the community (join the discussion here or here).
But in order to do that, they need artists and craftsponies willing to toss some of that aforementioned swag their way. Here's the bottom line: it's free to submit auction pieces, and 100% of the proceeds will go to charity. And even if you can't attend a convention to hand off your work, you'll still be able to mail it in.
So check out this handy link for all of the information you need to know, and then get to work. And if you're attending? Have fun!