Quite a bit. MegaTraveller shattered the imperium, New Era destroyed what was left, and switched to very different mechanics. T4 ignored it all, broke the system mechanics even further by taking the worst parts of the prior three.
T5 became vaporware.
Mongoose has a license for the next "real traveller" edition. It's also going to be used as a core ruleset for several IP's that they use, including Bab5, Judge Dredd, Strontium Dog, Starship Troopers, and will be open gaming licensed.
SJG has a licensed GURPS adaptation, there is a licensed Hero System adaptation, and you've found the home of the d20 adaptation.