Generally 20%. My girlfriend use to work as a waitress, and making a couple bucks an hour sucks. If I am remembering correctly she was making $2 something an hour, because it's assumed they'll be making tips. Tipping is where they make all, all, of their money. Anyone who is smug enough to think that waiting on someone is easy/well paid clearly hasn't done it, and need to not be so cheap.
If wait-staff was paid a good wage, then sure, tipping should be only for outstanding service, but wait-staff isn't paid a good wage. They depend on tips, and are taxed according to the presumption that they'll be making 15% of every receipt they bill as a tip.
I just get worked up about this, because I have seen how much it sucks to have people not tip. If you're really that cheap you should be eating Ramen at home, and not ruining someones night pretending you're too good to tip.
I will however, if the service is terrible not tip.