Well, pretty much every game I've ever played I'll be going for 100% completion but lose interest at some point along the way.
As far as actually completing a game, Portal is the one that comes to mind most often. Loved the puzzles and level design, but for some reason I just couldn't get through the game without getting distracted by another game. What's worse is that I actually got to the last level when I last played it.
I guess I could also put down Oblivion. Never did complete the main quest even after 100+ hours. I was too interested in doing other stuff, particularly the Mage's Guild story, to really focus on the main story.