Instructables.com is featuring a DIY Variable ND filter that functions similar to the popular Fader ND filers by LCW, Fader Filters, or or Genus Fader ND's. I've seen a few of these DIY's before using 1 linear polarizer and 1 circular, but this DIY calls for 2 Circulars.
The cost of a real Variable ND will run you about $110-$120 dollars for a 77mm, but by following the Instructables tutorial, you might be able to get a bit of functionality for under $10 dollars. The basic principle is attaching 2 circular polarizers together, but flipping one lens backwards. There's a bit of screwing and unscrewing involved, but pretty straight forward for those who want to test this out.
Cons of course are that they will block a bit more light (which is ok for full daylight) than a real Fader, dependent upon quality of Polarizers you buy might change color tone a bit, and with stacking of filters, there's always chance of Vignetting. I guess this is why the Real Faders cost money. If you want to try it out though, here's a list of Polarizers you can grab starting at around 99 cents and up.