how to get free hats on TF2?

Answer 1

if you play through steam alien swarm (free) you gain one for an acheivement, also others can be unlocked in tf2, people you know may also be able to trade them to youedit and the random drops for weapons may also get a hat if your lucky

Answer 2

Drops (after a trade or after dead), alien swarm parasite (game) achievement. Foundry pack achievements (TF2), Spiral knights (game) achievement, pre ordering games, and opening mann co supply crates.