Albert Rakowski has just released a 40-page PDF supplement to OD&D called Terminal Space. It's available for free and, having spent a lot of time reading it last night before bed, I can say that I'm very impressed. As noted, it's not a complete game but instead provides additional rules to expand OD&D into the science fiction and science fantasy genres. So it includes some new character classes, equipment, alien creature creation and starship rules, and more.
Reading it, I found myself amazed at how much material Albert had managed to pack into 40 pages. As someone who's written his fair share of RPG products over the years, including SF ones, it's more than a little humbling to watch someone else do so much with so few words. As with OD&D itself, Terminal Space is a masterpiece of concision and makes demands on referees and players alike to fill in the gaps. Fortunately, that's very much my style these days, so I have no complaints, but anyone expecting an exhaustively detailed, ready-to-run game may be disappointed.
I'll probably talk in more detail about Terminal Space later this week. It's a really amazing piece of work. I wish I'd thought of it.