Simple process to get extra experience while you sleep. I average about 1 million experience an hour.
This works best in Act2. I use the Vault of the Assassins quest. Easily done in Nightmare, Possible to do in Hell mode with good gear that doesn't need to be repaired and very high life regeneration. Inside the tomb do the following.
1) Find 2-3 Spider Spawning Statues close together.
2) Stand so that all of them are casting on you at the same time.
3) You will need some type of thorns damage, either from gear or skills. If gear make sure it is invulnerable and doesn't need to be repaired.
4) Set a macro or have a button held down to cast continuously so you're not kicked from being idle.
Life regeneration on gear that is invulnerable will help greatly. The kill streaks will continue to build while you're AFK so make sure you leave combat before you exit the game. *Also leave your follower in town.