World of Goo may be the best game I've ever downloaded. It is beautiful, thought provoking, and the most fun I've had for fifteen dollars. To think, the game was only developed by 2-3 people. This game reminds me a lot of Portal and I think any fan of that would do themselves well to pick this up ASAP.
I picked this one up last night, along with Tetris Party.
World of Goo is gorgeous, which you can't say about many Wii games. It's just ridiculously well designed. Everything from the look to the music to the addictive gameplay - it's obvious that a ton of care went into this.
As for Tetris Party, it's Tetris! With a ton of different game modes! I didn't try most of them yet - including the vaunted Balance Board mode - but I greatly enjoyed the one (Climber?) where there's a little man in the pit and you have to help him escape. I kept killing him but I still had fun.
$27 for both Tetris Party and World of Goo is a steal. Either one would be worth that by themselves.