15% is the minimum....Waitstaff only make $2.35/hr and are taxed on at least $5.15. Those who don't tip are only hurting the people who are busting their butts to serve them. I was a server for 5 months and have the upmost respect for servers.
Many servers also have to pay the busboy.

I always tip at least 20% unless the service stinks.

Don't forget they often tip out bartenders as well when drinks are served at the table. I agree with dorine, in addition to showing gratitude for good service, many waitstaff remember the nice customers (not just good tippers) which means even better service the more often you go back.

Even if you're visiting a restaurant just occasionally, treating your server right means that they'll keep a positive face throughout the night, which benefits everyone.

(for the record, I've never worked in the restaurant biz, but know/talk to a good handful of those that do/have)