There are (at least) two threads about this on the "Wine Conversation" forum as well. I believe one is called "Daily Drinkers" and one is called "Best wine under $25" or something like that. I remember there being some great suggestions in those, so might want to do a search...
But, I'll copy here what I posted there. My favorite sub-$20 wines are:
2004 Juan Gil Jumilla ($14) - I drink this quite often. Great QPR!
Dr. "L" Riesling ($10), the 2005 or the new 2006, both quite nice and refreshing, though distinctly different. This is my wife's favorite daily drinker.
2003 Poggio d'Arna Toscana Rosso ($12, $9/case) - I keep this stocked at the house. Fantastic stuff!! Needs a good hour or 2 in the decanter to get going, but this is as good as some Tuscans I pay $20-30 for, EASY!