I tend to cook with the two wines I drink most often, which I end up having leftovers of in the 'fridge. Usually these are Chianti and Pinot Grigio.
I buy cases of an Italian Chianti Classico I like for around $12/bottle, and my standard table Pinot Grigio is a cheap $7/bottle mass-market selection (Yellowtail). Oddly I don't like any of Yellowtail's other wines, but their Pinot Grigio is a good cheap wine.
Some specific dishes I will get a specific wine for however, such as Merlot with berries, or Chardonnay for butter sauces.
Nick ~ "Egg whites are good for a lot of things; lemon meringue pie, angel food cake, and clogging up radiators." - MacGyver