You can use MUSHclient under Wine, which is what I do when I do serious development. Otherwise, I use tinyfugue, mainly because there are enough things that irk me about tintin++ that I prefer tf. Finally, if tf isn't available, I use tintin++, and if that's not around, I use telnet.
There are Linux-specific GUI apps, but I don't really know any. The ones I tried (can't remember their names) were fairly basic, not much more than a GUI around telnet with trigger/alias support.
TinTin++ is probably the way to go if you're familiar with zMud, it's more powerful than tinyfugue while remaining fairly easy to learn, is multi-platform, and has an active forum if you need help with anything.
Papaya is still my favorite but it hasn't been maintained in years and is hard to find. I have the last source code release and I maintain the code enough to keep using it myself. This is a Gtk+2 client.
The next client I use is Kmuddy which is a good actively developed client for KDE4.
Finally, I use tintin++ on anything and everything including Linux. If I am at work I ssh to home and this is what I use.