I want a visual GUI designer that will produce XML output in the format used by GtkBuilder. Glade seems to be the most powerful, although Gazpacho is more lightweight. Stetic (included with MonoDevelop) seems to be good but I don't believe it supports GtkBuilder yet.
The last time I tried to use Gazpacho -- admittedly, a few months ago, so it might have improved -- it was nearly unusable. Constant crashes[1], couldn't set some important widget attributes, and to top it off, its final output couldn't be loaded. I ended up just using Glade, saving to a .glade file, and converting to GtkBuilder manually. I've heard that Glade's GtkBuilder output is better in recent versions, so you might be able to skip the second and third steps.
If you decide on Glade, make sure to use Glade 3 -- it's much, much better than the older version.
[1] Yes, I know it's written in Python. No, I can't figure out how they managed to pull off such an unstable Python app.