You will find a full list of all achievement rewards at Valve's Team Fortress 2 Wiki. Some are dependant on character milestones, while others rely on events that may be locked out. Several come as rewards for unlocking achievements in other games, on Steam.
Character milestone rewards are unlocked for completing a certain amount of achievements in the respective class achievement category. I have included a link to each achievement list in the class name.
- Blutsauger (rewarded for 10 class achievements)
- Kritzkrieg (rewarded for 16 class achievements)
- Ubersaw (rewarded for 22 class achievements)
- Hunstman (rewarded for 5 class achievements)
- Jarate (rewarded for 11 class achievements)
- Razorback (rewarded for 17 class achievements)
Halloween Rewards unlock by completing certain achievements within the Halloween maps and events. Most are more accessible during Halloween, but can still be unlocked throughout the year. Simply join a server running the appropriate maps.
Content Update Achievements
These achievements relate to content updates and various other additions to the game.
These items are earned through alternate Steam titles.
- Alien Swarm Parasite (earned by completing Hat Trick : "Complete any two missions, on any difficulty level")
- Black Rose (earned by completing 1st One Down : "Complete your first game")
- Iron Curtain (earned by completing Special Item: The Heavy : "Knock out The Heavy adfter he has brought in his special item.")
- Enthusiast's Timepiece (earned by completing Special Item: Tycho : "Knock out Tycho after he has brought in his special weapon.")
- Dangeresque, Too? (earned by completing Special Item: Strong Bad : "Knock out Strong Bad after he has brought in his special weapon.")
- Lugermorph (earned by completing Special Item: Max : "Knock out Max after he has brought in his special weapon.")
- License to Maim (earned by completing Special Item: Max : "Knock out Max after he has brought in his special weapon.")
- Long Fall Loafers (earned by completing Personality Goes a Long Way : "Win GLaDOS' bounty item.")
- Necronomicrown (earned by completing Book 'Em : "Win Ash's bounty item.")
- Samson Skewer (earned by completing Orb 'n' Legends : "Win Brock's bounty item.")
- Bloodhound (earned by completing Banjo Hero : "Win Sam's bounty item")
- Dapper Disguise (earned by completing Trophy Wife : "Win Claptrap's bounty item")
- Spiral Sallet (earned by completing Mission Accomplished : "Reach the first Clockworks Terminal")
- Flamingo Kid (earned by completing Rookie Agent : "Achieve agent level 5")
- Triclops (earned by completing All-Star Agent : "Achieve agent level 20")