The Ludum Dare events are pretty awesome, and it's been a worthwhile effort for me to take part. The themed 48 hours (compo) or 72 hours (jam) or even the mini LDs available to you makes for some gross efforts or super brilliant ideas.
"Mommy there's a monster" MiniLD 31 (January 2012) Second submission to LD48. This one forced me to learn animation in XNA within a few hours, and it turned out pretty well. Download here to play. Requires .NET 4.0 and XNA 4.0 with DirectX 9c or later.
"Speed to Freedom" Ludum Dare 22 (December 2011) My first entry to LD48. Had a ton of fun joining the in-person session with other members of the Cape Town community. Download here to play. Requires .NET 4.0 and XNA 4.0 with DirectX 9c or later.