I also found some great ideas here and there, like:
There are various things you can do.
Use them as basic level knitting needles.Use them as vials for small glass beads
Sometimes heating / tapping them will get the ink to flow again.
Use them as straws - make sure to cover the tiny vent hole in the side.
Use them to blow paint blobs around on paper for art.
Various poking uses (planting seeds, holding up seedlings, pushing wax bits into the melted pool of candles, etc.) -Amanda Kerik
Cut the outer tubes into beads and make a bead curtain
Glue an info card to them and use as plant stakes
String together and use as a fan/light pull
String them together to make jewelry
Or you could always make them into a chandelier (below):
If you're impressed by that, you might want to check out some of the other eco buys.
Like always, instructables.com has tons of great ideas. Everything from pen blo guns to pen safes. So, while there are many ideas on what to do with the pens shells, about the only thing I couldn't find was how to keep the cartridges out of the landfill.