To Heart (Remember my memories)
To heart 2
Serial Experiments: Lain (very weird)
Ichigo Mashimaro (check out the manhole release at www.manhole-fansubs.com)
ad a host of others - I know all those can be gotten via bittorrent - but you may wlaso wantto check out your DVD supplier too.
For animation and creative storylines, ANYTHING by Miyazaki: in addition to Princess Mononoke and Spirited Away, try Porko Rosso, Nausicaa, and Castle in the Sky (Totoro and Kiki's Delivery Services are targeted for little kids, but the animation is spectacular). His latest, Howl's Moving Castle, comes out in March.
Cowboy Bebop probably my favorite series, and has some of the best music. Trigun's pretty fun, too.