I have also played TikiTowers-- fun game, not nearly as good as World of Goo, but it has some fun ideas such as bannamana and using magic spells to try to stop evil monkey's. I found it a little repetative toward the end-- but by that point I had played it for about 5 or 6 hours (I was going for medals on some of the earlier levels before I beat the game), so I believe it is well worth the 500 points.
I think Tetris is a great steal at 1200 points-- so many modes and online multiplayer. However, it is tetris and it has been around forever. If you enjoy puzzle games, Dr. Mario is fun.
Some other recomendations: Art Style: Orbient and Lost Winds (this game is only about 3 hours, but after a hard day at school/work I find playing this game for 15-20 minutes is a great way to unwind-- this game is very relaxing and very well done). Bomberman is fun as well.
Megaman 9-- If you are looking for a challenge, this is it (I also think it is better than the other megaman games-- just don't buy any of the DLC-- most of it is a ripoff).
I see some others mentioned the Cool Bad games-- I personally did not like them, but if you are a fan of the online comics then you can't go wrong with this.
