Using Robert Penner’s easing equations is probably one of the easiest things you can do to spice up your UI. Apple’s famous for using CubicInOut almost everywhere they can to make their interfaces look sleeker. The only problem is, it’s hard to pin down the exact equations if you need to implement them yourself, or get a good demo to compare which one you want. Tweener, which implements the easing equations in ActionScript, has a great demo of all the different easing equations. Probably the best reference I’ve seen for objectively looking at easing.
Codepedent has a great explanation of Penner’s equations and how to use them.
Gizma.com has the actual equations (although the site is supposedly dead)