Doomworld, as usual, is hosting the annual Cacowards, too; be sure to check them out and play this year's winners if you haven't done so already.
And while you're at it, celebrate DOOM's birthday by playing through the original levels! If you don't have it, grab a copy of the shareware episode, download a source port so it'll run on your PC (I'd recommend Chocolate Doom if you want something as close to Vanilla, PrBoom-plus if you want a Boom-based port, or GZDoom otherwise; alternatively, you can run the original in DOSBox, too).
Lock and load, marine! >:) Don't leave yet — there's a demon around that corner!