Revenge of the Titans and Cortex Command make it worth it.
Those are the only two I want. I already own Braid and I played Osmos and Machinarium before, but didn't like them enough to want to buy them.
EDIT: Out of pure curiosity I put in $0.00. It worked until PayPal was all like "You need to actually enter a value."
EDIT2: I've been playing Revenge of the Titans and Cortex Command. Revenge of the Titans is probably one of the only Tower Defense games I've played and actually really enjoyed, and Cortex Command is just a blast. I know it's "In Development" but other than the Campaign Mode that's not there, I really don't see much in the way of missing features. I mean heck, there's built in editors for darn near everything (I need to get on making some maps at least). My only complaint is the font is anything but legible when you're playing at anything larger than 800x600 resolution. Oh, and multiplayer is local, which isnt a bad thing, and I love the integration (It even picked up my random 3rd party USB Sega Genesis controller, and my Joystick of all things) but internet play would be nice. Otherwise, I'm having a ton of fun with it. Good thing I'm on break right now or my GPA would be tanking, between this and AI War