Now, the usual solutions that come to mind are OpenOffice.org and Abiword. They look obvious, but they don't really work. Believe me, I've tried. Like a tease, Abiword will let you create an envelope sized document, but when you try printing it, you'll be out of luck.
Actually, glabels should fall under the category of a graphics program. It gives you tight control of how you print on your document. Text, for example, can be any size, any font, and any orientation.
More than that, it comes ready with several hundred preprogrammed label options for different kinds and different sizes of gum labels. Very nifty.
But as to printing the envelope itself? glabels doesn't have an envelope document, per se, but you can simply position your label on a regular letter size paper (some trial-and-error required) so that it comes out right when you finally print it on the actual envelope.
Simple, yet neat!
Tested this on an Epson C90, by the way.