Hmm... let's see :
1. The total war series discount from steam sales - never played it, although the 2.50 price tag doesn't hurt that much.
2. The indie bundle from steam X-mas sales.
3. Starcraft 2 - I guess I never liked the game. gave away my account in the end.
4. Assassins creed - BAD PORT. Same thing happened with this in regards to SC2.
5. Monster Hunter Freedom 2nd G - OS update, thats all I will say, also trading your old game scheme is just horrible.
6. The huge amount of Game boy advance games that I gave away. Those things were good for their time when I don't have to pay for my games, but today I see it as a huge waste of money.
7. Black Ops - What else do I need to say about its first few weeks upon its release on PC? And then they had it on sale at Steam X-Mas.