I always tip waitstaff at sit-down restaurants where someone brings the food and drinks to the table, comes back to check on us, etc. Fast food places where you don't really have a waitress, no. I'm not sure who made up the rules... I'd really prefer to just have everything built into the cost so there's no guesswork, but it IS nice sometimes to be able to tip according to the service given, if it was extremely wonderful or utterly lacking.
Other services I tip for would be taxi rides, haircuts, bellboys or valets, street musicians and party entertainers. I'm a party entertainer myself, and though it's rare, some people do give tips. I never expect tips, but I've had people insist that I take them based on how pleased they were with my skills or services.
It is odd that some service professions get tipped and others don't. I don't really know what the set "rules" or expectations are. Many are just unclear.