I have been intending to write something just like Quark for a while. The music player is a daemon that lives on even if X dies. The GUI for it is just a tiny icon on your panel that lets you enqueue files.
I wrote a half-assed OSD babbler for quark (connects a FIFO, parses the output and puts some of it into an osd call), but to get the full range of xmms' OSD plugin, I'd need to hack quark proper to be more verbose. This possibly requires moving functionality around. (I need to check where strange-quark's mouse-wheel volume stuff is implemented.)
Uh. I knew about that wheel thing, honest. Wasn't confusing it with rhythmbox or xmms at all. Yeah, that's it.
I guess the thing that I wanted to do must have been an indication of whether you're skipping back or forward in the playlist when you do skip, and being able to differentiate that from a normal song change. On screen volume is probably done by at least a dozen other implementations already ...
I got gdesklets working all prettily last night, and while I'm impressed by it about as much as I am by gkrellm (not at all), the starterbar is kind of nifty. All I need to do now is figure how to get ion to stop putting titlebars on all the pretty windows. :P
(It's python and uses 25% CPU on my old K6-2/500 but seems to use negligible resources on the Athlon.)