Gundo is probably the coolest plugin made for Vim. Changes in Vim are tracked not in a simple linear fashion, but the entire tree of changes is maintained. So, you can actually undo back to a fork in the tree and change track to go along a different path taken earlier. This is fantastic, except it is unusable in practice since there is no way to visualize this.
Enter the fantastic Gundo plugin by Steve Losh. Install it using Pathogen and you are set. Anytime you want to see the tree of changes in Vim, use the command :GundoToggle to see a ASCII tree of changes to the left. Move up and down the tree, see a preview of the changes below and press Enter on a change to apply it live to the code to the right. You will not believe how awesome this is until you use it.
Tried with: Vim 7.3