FCPX must have!
I'm still not a fan of FCPX but I'm using it for most projects now although I still have some FCP7 projects on the go (that's a long story) but this is a must have if you're using FCPX, and it's free!
Best thing for me? SCROLLING TIMELINE!
There's many other useful tools that should have been included in FCPX - Moving Markers, Custom shortcuts, Clipboard history, Timecode overlays and lots of other cool stuff, it's certainly going to make me more acceptable to using FCPX.