Parcellite
A while ago I wrote about Glipper, but I seem to be affected by bug #213494, which means that about half the time Glipper crashes on startup; it didn’t use to do that.
There’s a proposed workaround on the linked bug report, namely to add a timeout to the beginning of the Glipper code. It seems to work alright for the most part, but Parcellite offers the same functionality without any of the downsides: it doesn’t depend on Gnome Panel and, of course, it doesn’t crash.
The easiest way to get it is, as always, sudo apt-get install parcellite. A more recent version can be obtained using the Web Upd8 PPA.