Toonami is some of the best stuff on Cartoon Network, rivaled only by Dexter's Laboratory. As noted above, Toonami is a time block on Cartoon Network which shows mostly anime.
In addition to the afternoon block of Toonami (which runs from 4-6 every weekday), Cartoon Network shows Toonami: Midnight Run weekday and Saturday at midnight, and Toonami: Rising Sun on Saturday mornings
Some shows featured on Toonami, past and present: