Trackbar Gesture for Mission Control / Spaces: Two fingers + click
On a trackpad:
slide with 4 fingers up to open Mission Control
slide with 4 fingers to the left or right to switch to another 'space'

On a magic mouse:
you can dubble tap with 2 fingers ( don't click ) on your mouse to open mission control

But assuming you have a mighty mouse ( your nickname ) you can assign Mission Control to a hot corner. Go to System preferences -> Mission Control -> and press the hot corners button to change or assign the hot corner you want.

Also you might want to check out BetterTouchTool which let you define al sorts of gestures on your magic mouse, normal mouse, touchpad and or keyboard. Assigning them to whatever you want. ( like opening Mission Control )