Escaflowne (about half the series)
Hellsing (not sure, maybe 1/4 - 1/3)
Neon Genesis Evangelion (full)
Cowboy Bebop (full)
I got some pretty good deals on eBay on most of the full series. It turns out that it's convenient that I prefer subtitled stuff as opposed to dubbed versions. Across the board the subtitled stuff is much cheaper than the official US releases (assuming they have been released in the US in the first place).
I also did some research on DVDs recently and I figured out why I kept seeing so many regionless (i.e. Region 0) DVDs on eBay. Apparently those are probably legitimate Chinese copies. Apparently until recently most Chinese DVDs were produced without using regional encoding so it's possible to buy those for use in the U.S. Unfortunately this trend is diminishing lately, but a lot of DVDs produced in China and other asian countries are still made regionless. Nifty.
I'm also seeing that you can get a lot of hard to find stuff on DVD if you don't mind buying a foreign release. Recently I picked up a (cheap) Chinese copy of "Desk Set". It's in English with optional Chinese subtitles. Considering that it was about as expensive as the VHS copy available at amazon.com (and that there is currently no US DVD release) that's a pretty good deal.
Hmmm just to keep things together, here's the list of things I want to track down in the near future. I haven't seen much of some of them, but I've read about them and they look good.
Escaflowne (2nd half)
Hellsing (whatever is left)
Gundam (how many series are there?)