Free tasting is almost unheard-of at Napa Valley wineries these and every the list grows smaller with time. If free wine tasting is what you're looking for, you may be better off to head for Sonoma instead, where almost half the tasting rooms don't charge a fee. The few Napa wineries with no tasting fee are hard to keep up with and the folks at American Winery Guide do a better job of it than we can, so your best resource is their guide to Napa wineries with free tasting
Wine Tasting Discounts
Many wineries will also refund your tasting fee if you buy something.