As far as I know there are only three: A single-disc DVD, the Millennium 2-Disc set, and the current Anchor Bay 2-disc special edition.

Here's my count.

1. 2001 edition 1-disc (Elite)
2. Millenium Edition 2-disc (Elite)
3. Anchor Bay Collection 2-disc (Anchor Bay / Starz)
4. Anchor Bay Collection 1-disc (Anchor Bay / Starz)

optional

5. Anchor Bay w/ exclusive highlighter pen
6. Anchor Bay w/ exclusive pen that looked like a vial of reagent

I think that's it. I could swear I saw two different releases with two separate exclusive writing utensils.

What he said.