I have to second Mushclient. Few minor issues:
1. Its scripting isn't zMUD based. Originally, most of it was in vbscript and jscript, because that was what was installed on the machines. Later Python and PHP where added (though PHP is flaky of you don't have a recent version). Now its 'default' script system is Lua, with even some special internal functions that only work with that language. Some people have made converters for zMud style scripts, though mostly just triggers. ZMud's system is, in some ways, highly simplistic. Unfortunately it also uses a slightly different way of handling incoming information. Mushclient processes a line 'after' a newline, zMud and most others process it as it arrives. This leads to some minor complications for some times, though there are ways around it and recent versions also recognize a code sequence used to ID the end of a prompt, so can convert that to a newline, so the prompt is read.. promptly. lol
2. It isn't a free client. It will work forever without registering, though with a nag screen. As it is, probably 90% of the people that have it either a) don't register or b) hack it. Imagine for a moment if your mud requested a small donation to prove you where 'serious' about playing, but 90% of the people didn't bother or found a way to hack their characters, so they didn't get asked again. What would that likely tell the people developing the mud? What if that was thousands of people? Why does no one seem to understand this?
3. No mapper. Now, there is some progress on this. Someone has been working on something that can display pre-existing zMud maps and have the client tell the displayer where they are going on that map, based on the commands sent. It was made to be universal to various clients, though I am not sure how many of them allow their scripts to talk across worlds/characters, since it uses a goody TCP/IP setup, which requires two windows to be open at one time, one to the mud, the other to the mapper. Other more Mushclient specific methods exist, which he either a) didn't know well enough to try or b) didn't want to use, since the mapper wouldn't work with others. I don't think in most it will anyway, but... Now, the 'real' problem is, for those that support ActiveX, there 'should' be a universal solution. The problem is, its obscure, slightly complicated and I am not sure zMud supports it any more than Mushclient. I am relatives certain others like TinTin++ and SimpleMU absolutely don't. I'm personally working on trying to figure this thing out, then convince Nick to impliment it. He doesn't have the incentive to bother himself, I don't really have the skill, but no one else is going to do it.
If I do manage to figure it out, then it should be possible to not only handle 'all' ActiveX type stuff correctly, but even build things like mappers, etc. directly in the client script. You would simply open a window in scripting, drop the buttons, etc. you need into it, connect events from them to your script, and away you go. Even zMud, last I checked, didn't have that flexibility. But, that's a 'future' feature and dependent entirely on 'if' I can figure it out, so I can show Nick how it needs to work.