[SnowTroll]

Good old Ardwolf, #1 on the list, has a good selection of races, each unique from each other, and an absolutely massive multiclass/remort system, where once you've leveled up in each class, you start the process all over again at the next "tier," slightly more powerful. I think you do that seven times before you're "maxed out."

Dark and Shattered Lands is a somewhat RP-enforced mud that has a sizeable selection of races (mostly Dragonlance based), and five base classes, but once you've reached a certain level, you have the option to change into a specialized class.

4Dimensions has a small selection of races that are mostly similar to each other, with a tiered class system where you start out as a base class and pick a specific branch of that class later.

End of Time is a Final Fantasy based mud with a good smattering of the races you'd expect if you're familiar with the console games, set apart by unique racial abilities. The game's still in development, but slated to go "live" any day now. It has four base classes, then at certain levels, you choose from various specialized class options.

That's just a few off the top of my head. Really, there are a lot of muds out there that have a wide selection of races, and a system that lets you change/specialize your character class at one or more points.