The original was 1.0, then 1.1 and now 1.5 which is Cupcake.
If you go to
http://forum.xda-developers.com/forumdisplay.php?f=448 you will see all of the various "hacked" versions.
Basically the fall into about 4 categories.
Original Cupcake version 1.5 (True Dream Version)
The below are Hacked versions of different Phones, hacked "root" versions to work on G1.
Ion: Like 1.5 but very fast.
Rogers: Canadian Version.
Hero: Very pretty but very slow and buggy.
I don't think Hero will ever really run on the G1 because the hardware differences. The Hero would be a nice phone to have, but it is $$$ and not available here yet.
The Rogers was only recently hacked to use Root.
Ion (the one I use) very fast and very reliable. Pretty screen, I currently use Soullife's Ion Mod.
Original Cupcake is nice if you want to stick to an unrooted phone. There are plenty of good apps to include Themes.
By the way, you should see the traffic on that site. Its no BS forums with Hard love so don't feel down if your thread gets removed. Search all the threads before you post a question. You would be surprised how many questions are repeated hourly.