It was pointed out that even a free lunch isn't ultimately free. This is generally true. Every meal you take, borrow, or steal probably takes something from some other poor bastard.
This writeup details a few of my favorite methods of obtaining free food. I developed the methods before reading Abbie Hoffman's Steal this Book, but he's covered more methods, in better detail, than I ever could. Read Steal this Book. Thank me later.
For the Really Desperate
At the end of the day, most bakeries get rid of their unsold, stale bread. If you don't make yourself too much of a nuisance, you can usually convince an employee to set aside some bread for you, or just pick up any bread they toss in the dumpsters behind the bakery.
An increasing number of bakeries donate their unsold bread to local homeless shelters, so the only bread they toss out is too stale to eat except in cases of extreme desperation.
For those Who Like Acting
College campuses are a good place to pick up free food. Your ideal campus is one which is separated from the city, the type of place where security isn't constantly watching out for nefarious types sneaking around.
Show up on campus at about midnight. Find a few frat houses. Wander inside, make friends, and help yourself to any appetizers you see. Try to seem just drunk enough to avoid much socializing, just wander, smile, laugh, and eat.
I don't really like this method. Parties don't tend to have much food, so this technique works more as a novelty "just to say you did it" type thing. If you actually need food, these other methods work much better.
For those Who Feel Fancy
Find a hotel. Nicer hotels are harder to use this technique on, but have better food. Arrive early, no more than an hour or so after they've started serving complementary breakfast. Make sure that your clothes match the style of your hotel - fanny packs and shorts for your typical Holiday Inn, business clothes for your typical Marriot or Hilton.
If you're questioned, you can get out of most trouble by leaving to "get your key". The security doesn't care what excuse you use to leave, their only concern is keeping undesirables away from the guests.