Also, remember: there are LOADS of editions...
Official and Authoritative about the OTU:
CT aka LBB's, Classic Traveller.
MT, MegaTraveller, sometimes called MegaEratta. Mostly compatible with CT.
TNE, Traveller the New Era, Virus Era. Not compatible with any other Traveller edition.
T4, AKA Mark Miller's Traveller, MMT. Design systems derived from TNE. CHaracters and stats not compatible with TNE nor CT/MT
T20, AKA Traveller's Handbook (only setting authoritative)
Mgt, AKA MoTrav, aka RTT, aka Ricki Ticki Traveller, Mongoose Traveller.
T5, aka Vaporware Traveller. The "Any week now" version that was promised for LAST year, and still ain't out. Somewhat T4 compatible
Official but non-authoritative:
GT, GURPS Traveller, snarkily Loren's Traveller (for Loren Wiseman). It is an Alternate Traveller Universe, albeint one very close to the OTU. All equipment and characters incompatible with other traveller flavors
HT, H4T, HeroTraveller. All equipment and characters incompatible with other traveller flavors
Not OTU but labelled Traveller:
T2300, renamed 2300AD. Conceptually similar to CT/MT and TNE; almost bridge like...
2320AD (T20 supplement for the 2300 universe). T20 with tweaks.
Not Traveller, but useful for TNE, as the system's the 98% the same:
T2K, Twilight 2000 (2nd edition).
Dark Conspiracy
Cadilacs and Dinosaurs
Related system which is a precursor to T2K2E (which is the precursor to DC, which leads to TNE):
Space 1889 (S1889)
Unofficial conversions on the net (One's I've actually read; I wrote the first one)
Storyteller Traveller
AD&D Traveller
CORPS Traveller
The Fantasy Trip Traveller
FUDGE Traveller
1PG Traveller
Fantasy Traveller (several versions)
you'll find fans of all the above here on COTI. (Well, no one has claimed fandom of the AD&D traveller since 1999 or so... and I don't know any who have for HT...)
In print on dead tree: CT, MgT, GT, HT, T20, DarkConspiracy*, Space 1889*
In e-print: CT, MT, TNE, T4, 2300, HT, GT, T20, T2K**, MgT***
So, you can find just about any edition CURRENTLY
*Last I checked, at least.
** 1st ed, 2nd Ed, and 2nd ed revised. 2.2 is the most TNE-like.
*** well, not yet, but it's been announced that it will be.