Well, what do you know. I was recently gently and not at all forcefully convinced to try out a MUD again. Last time I played MUD's was in 1994 or so. MUD's will be very strange to people not used to them, having very simple text interfaces, no sound and no graphics.

Anyway, Shadow Siege (shadowsiege.com) is definately one of the better, and I've been impressed by the content and systems built into it. It's heavily RP, you'll not meet any non-RP'ers here. You get points to spend on skills based on your level of RP. This is done by a system that detects spelling, use of emotes, punctuation, amount of OOC used (kept in seperate channels), how many people you interact with and so forth. It works wonderfully, and it is refreshing to actually not need to grind, but be rewarded for RP.

I'll mention some unique things about MUD's and Shadow Siege here, so you'll get a feel for whether this is something you should try. Since there're no graphics, MUD's are for people able to get immersed in text only naturally. Also, the initial learning curve if you haven't played MUD's can feel steep, there will be many questions, as you have to learn a lot of commands in order to play your char fully. With people answering your questions as you play, this shouldn't take more than a day (I'll be glad to help naturally, as will others).

Playing a MUD also takes a little more patience in many ways. In Shadow Siege for instance, you do a fair bit of typing, explaining how your char moves, behaves, looks and what he or she says, every time you write some input. This is more cumbersome and slow on one hand, also waiting for others who might not be fast typers, but on the other hand it's a great way to sharpen up your prose, getting better at expressing yourself, making your char come to life more. 

When it comes down to it, I personally find that RP can be found in any medium where people want to RP. The MUD/MUSH medium has some advantages to MMOG's or RP outside the computer, and of course also vice-versa.  MUD's are slower, take more imagination and dedication to get into, but the experiences you get in them are more constant, and can very quickly suck you in completely if you find the right one and in general can get enjoyment from MUD's.

As for Shadow Siege, this is pure RP, without the grind and OOC. It doesn't have as many players online as a MMOG of course, but with 20 RP'ers in a city, that can easily seem better and bigger than hundreds of OOC'ers. And of course there are GM's and real NPC's, plots and intrigues.

If you want to give a MUD a try, Shadow Siege is surely a good one to try, also because a few of us play there. It's a good idea to read the info on the shadowsiege.com site, and then write up a background, sending it to a GM and getting a telnet client to log into the game. If you need any kind of help, just tell me in PM or catch me on the IRC.