For a simple cake, the Hershey black magic cake:
A little more complicated but worth the effort, the cake from this triple chocolate celebration cake:
Both are sturdy enough to hold up to decorating/mousse/etc.
Epicurious has the Barrington Brewery's Chocolate Stout Cake. It's fabulous! I've made the full recipe as a 3 layer cake (it's very tall, so beware), halved it and made a 2 layer cake (which is much more manageable for hubby's birthday when it's just the two of us eating it) and used half or whole recipes and done it as cupcakes.
it's definitely dense, but moist and good. It's my go-to chocolate cake. Here's last year's birthday version: http://web.mac.com/gfiorillo/GFB/Phot...