An exercise for guitar players that promotes finger independence. The simplest form is shown below in tablature; once you become somewhat proficient at playing it, start skipping strings, ie bounce back and forth between the E and D strings, then E and G, etc. The most effective way to play it is using alternate picking, and of course you should always use a metronome when doing any repetitive guitar exercise so you can work at getting faster.
E|--------------------------------------------
B|--------------------------------------------
G|--------------------------------------------
D|--------------------------------------------
A|---3---4---1---2---3---4---1---2---3---4---1
E|-1---2---3---4---1---2---3---4---1---2---3--
etc...