Lately, I've been using this saddle bag from Ortlieb. It's a simple waterproof roll top style bag with bungees to tie it down, detachable mounting system for easy removal if you're afraid to leave it on your bike when it's locked up.
its spacious as I've got a pair of tire levers, c02 inflator, multi too, and spare tube with plenty of room to spare for another tube, golf ball, energy bar, or maybe an Iphone.
Here is how it mounts to your saddle.
It also comes in these other colors if neon lemon isn't your fancy.