Console editors, such as emacs and vi, are more lightweight than their GUI counterparts, and (at least those two are) just as capable as any other IDE (syntax highlighting, mouse support, ctags, autocompletion ... all the way to gdb integration). The learning curve might be somewhat steep, and you might have to do some customization, but its all worth it. Also, vi is present on every installation of unix-like operating system.
Amongst X applications, there are
gedit which comes with GNOME and has many of these IDE features (see, for example, this blog entry),
Geany - really fast, depends only on GTK, and with even more features including code folding.
These would be lightweight IDEs, as opposed to heavyweights like Anjuta, KDevelop, Eclipse or NetBeans.