There's a good discussion about designing timelines and other handy visualizations over in the Tinderbox Forum.
Here's another kind of timeline:
Timelines lend themselves to chartjunk, but sometimes it's nice to organize your notes so you can see quickly whether the Peabody sisters had snowball fights with Henry Adams, or who knew what when.
Have ideas for other useful visualizations? All this started from a quick scan of a sketch on the back of an old calendar sheet. Contribute your own ideas; who knows what will arise?