Good cru Beaujolais (e.g., Morgon, Julienas, Moulin-a-Vent, et al.). The 03s were good, but you're never far from a good year if you stay with artisanal producers.
Cotes du Rhone is a reliably good deal. In fact, Parallele 45 is consistently one of the best $10 investments you can make in a bottle of red wine. Some of the higher-end Cotes from smaller producers (e.g., Tardieu-Laurent) are good buys, too, and won't generally crack $20.
Occasionally I dip into southern Italian wines. Some interesting experiences, but I can't tell you producers or even regions that were especially good.
You used to get some good minor Northern Rhones for less than $20. Graillot's Crozes or St-Joseph, for example. Those have gone the way of the overvalued dollar.
There will be some good minor Bordeaux for under $20 that can, in good years, give happy drinking. 2005 may be a year to back up the truck on those. Otherwise, some of the Grands Crus in minor years like 1997 can be excellent value. That year got crushed by overpricing as futures and merely good reviews. Talbot may be available for around $20, or Grand-Puy-Lacoste.