the starter rules for classic Traveller have been released as a free download from RPGNow. Note that although they're described as "Starter" rules, they're by no means incomplete; these are, to all intents and purposes, the complete original rules plus some material from later editions.
There are also a bunch of supplements available for a couple of bucks each — Randall at RetroRoleplaying has links to some that he thinks (and I would tend to agree) are essentials.
Unfortunately, the PDFs are all just collections of scanned pages — they haven't been OCR'd, and the scanning quality is a bit variable. From what I've seen so far they look as though they're generally readable at least; I haven't tried printing any yet. If I get really enthused, I might try OCRing them myself so that I can reformat them to A5 and get decent-quality printouts for myself, but that will take a bit of effort, and I'm not (yet) sure that I can be bothered.
Hero System, using their excellent Star Hero and Terran Empire supplements, but I adopted several of the tropes popularized by Traveller into my Hero campaign.
I'm also a huge fan of Joss Whedon's Firefly TV series, and the movie Serenity, and have for some time had a growing urge to run a campaign based, if not actually on Serenity, at least in that milieu. Traveller would be a perfect fit for it, I think, as it (as I remember the game) always seemed to concentrate on the seedier underbelly of a shiny technologically-advanced future.
I'm quite looking forward to actually reading the rules for myself, as I never owned a set of my own when we were actually playing the game.
Postscript: I printed the Rules and Charts & Tables booklets digest-size, and they printed out just fine. The text prints a little coarse; it would have been better if it had actually been text rather than a bitmap of some text, but I guess beggars can't be looking gift horses in the glass house. Or something. The originals appear to have been formatted with fairly small type on letter-size pages, so the text is pretty microscopic when shrunk down to digest-sized pages; I won't be reading much of it without my glasses I suspect.
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