Most of us probably have seen this picture at one time or another. If you haven't I suggest you download it (4 megs in size!), and start searching. How many anime's can you find? Supposedly this picture contains almost _all_ anime characters up to the mid 90s.
Since ImageShack allows up to 1.5 megs, and PhotoBucket allows up to 800x600 pixels, this picture was dropped to some other image hoster. I have no clue if it works the way they advertise. It's not resized, so to prevent insane page-loading times, I'm just including a link.
http://files.photojerk.com/Neko/1157837262285.jpg
Or alternativly: http://studioshinnyo.com/war/images/ZOMGANIME.jpg
Just to get things going I picked out the ones I could find on first glance... I wonder how many we can come up with combined.
Robotech/Macross
Pizzacats
Kiki's Delivery Service
Porco Rosso
Galaxy Railroad
My Neighbour TotoroTenchi
Ranma
Fushigi Yuugi
Ah My Goddess
Slayers
Record Of Lodoss War
Neon Genesis Evangelion
Project A-Ko
Dirty Pair
Gundam Wing
Akira
Urusei Yatsura
Sailor Moon
Doreamon
Patlabor
Hello Kitty
Guyver
Astroboy