Lately there has been a flood of cheat sheets on various blogs. I bookmarked these and added ReSharper one I get asked about a lot:
My personal favorite is definitely RegEx.
I use regular expressions once in a while and don't have them 100% memorized. Just a little nudge helps a lot.
If you have more, post in comments, I'll try to keep this list growing!
Update #1: fixed Resharper link