Belated, as this one's been going around the web since Nov 2004... Running cheap-ass nasty vodka through a common household activated charcoal water filter clears out enough of the impurities to make it not actively bad.

This has nice implications for household brewing, as the vodka on my liquor shelf at the moment is distinctly nasty-ass (the cheapest stuff at the store), and it would be nice to have the raw ingredients for certain of my brews be of better quality, even though I do let them age.

All I need is a good filter pitcher and a few extra filters, and I should keep one of those around the household in any case because of the bleedin' Phoenix tap water that most people can't stand.