I spent a good number of hours fiddling with GUI things but the more I researched the more I realised: what a huge topic!
Some interesting reading here from KaVir as he documented the development of his GUI:
www.mudbytes.net/forum/comment/47814/
Its built for MUSHclient or Mudlet, and you can download it and alter it for your own grim designs if you wanted.
I also poked around with a GUI template available for Mudlet, again you can fiddle around with the code to display however you like.
forums.mudlet.org/viewtopic.php?t=4098
For myself I got a bit frustrated learning how to position the GUI elements and get text displaying correctly. I went through hours of trial and error to find that Stock TBAmud didn't have the MSDP code correctly patched in at that time haha..
Mudlet also doesn't support HTML which was a shame because I really like the overall interface.
Mushclient was able to do all kinds of cool things HTML-wise but I found the interface a bit clunky.
But.. its something I will undoubtedly pick up again soon.
Tintin++ and Tmux are two clients I've never ever tried, I'd love to hear what you think of their potential for more GUI shenanigans?