If you have you forgotten most of your algebra, Algebra Touch will refresh your skills using gestures adapted from the iOS version of this app. It lets you enjoy the conceptual leaps of algebra, without getting bogged down by the tedium of traditional methods.
Say you have x + 3 = 5. You can drag the 3 to the other side of the equation.
Algebra Touch allows you to create your own sets of problems to work through in the equation editor, and have them appear on all of your devices with iCloud. (accompanying iOS app required)