OtherSpace Interactive SF Saga
Otherspace is a science fiction MUSH which has caught and kept the interest of at least one player - myself - who would not normally consider involvement in online sci-fi gaming.
Its designer seems to go out of his way to have his creation meet and conform to just enough stereotypes and "rules" of the genre to be readily accessible to anyone with a basic understanding of almost any form of science fiction, before letting his imagination loose.
The setting and game which have emerged from this process are diverse, often intriguing, frequently entertaining, and (a rarity in science fiction) only occasionally confusing or contradictory. Greatly aided by the efforts of a number of active staff members and long-term players (many of the most prominent characters are run by people outside the admin team), Brody does a good job of ensuring that there is usually something significant going on somewhere in the MUSH each evening.
The diversity of the setting and the range of activities encouraged within it help to foster a broader array of characters and styles of role-play than is common to most other MU*s I've encountered. While Otherspace has a number of classic "heroes", an array of mercenaries, and a guild of bounty hunters, some of its more notable characters are medics, scientists, merchants and diplomats. And, in contrast to many other games, it's entirely possible to avoid risking life and limb should you wish to do so - my own character has been injured just once in over a year of game play (and that came from arguing with a martial artist). I also know neither in- nor out-of-character how to pilot one of the coded space ships (which are very popular among OS's fans of trading and combat simulation games), yet have never encountered much difficulty as a result.
OS's chief problems come from its merits - few people know all of the setting, and occasional errors are made as a result. It's possible to play for a long time without ever coming into contact with some sections of the PC and NPC population. And it can sometimes be hard to keep track of just what is happening where before you blunder into something you hadn't realised was taking place. But all of these can usually be overcome with a little reading of the news files and bulletin boards, or a few polite queries fired in the direction of the (nearly) ever-present Brody.
In summary - Otherspace is overall the best science fiction game I've come across online. It is usually friendly and welcoming to newbies and old-timers alike, its creator is notable for his willingness to listen to (thought not, of course, always to accept) alternative views, and the player base contains a large number of talented roleplayers portraying a broad array of characters.
I'd recommend giving it a try - if nothing else, take one of the pre-made "extras" (some of which are former PCs) out for an exploration of the setting. Just be nice to any black cats you encounter. ;-)