OtherSpace Interactive SF Saga
Well, what can I say that hasn't been said yet?
Basically, I just want to stress how awfully addictive OtherSpace MUSH is. I have only joined back in October, and the two months I have played were more than enough time to 'get hooked'.
Despite having never played in a MUSH or MUD before, I was so fascinated by the world I had stumbled into that I needed but two hours to learn all commands I needed to start playing - and from there, things went downhill. On numerous occasions I stayed up until six in the morning because there was RP going on I didn't want to miss. (I'm European and have to deal with a six hours time difference to EST.)
Speaking of RP: like all the reviewers before me, I can only praise the OS staff for the Story Arcs they run. Just like real GMs they come up with the beginning of a plot, some events that will take place, and a possible end - but it's up to us players whether or not the plot will really take the planned course.
All in all, it comes down to this: if you love original Sci-Fi, then you should visit the OtherSpace page at www.otherspace.org and take a look around. You will not be disappointed. ;)