Very nice tool I recently discovered: ink, a small tool using libinklevel, used to query the ink level of your printer (USB or parallel). In my case this is an Epson Stylus DX4200 (which works very nicely and out of the box btw).
$ apt-get install ink
$ ink -p usb ink v0.4.1 © 2007 Markus Heinz EPSON Stylus DX4200 Cyan: 76% Magenta: 76% Yellow: 76% Photoblack: 72%
Graphical ink level display:
$ apt-get install qink
Another nice tool built upon libinklevel is called qink, which is a QT-based GUI which displays the same information graphically (see screenshot).
A list of printers supported by libinklevel is available.