How about Medal of Honor: Frontline and Medal of Honor: Rising Sun? I feel like I always see those games at Goodwill or resale shops for $2 or $3 bucks each. They are both amazing games, but they were hugely popular when they came out, so copies are everywhere. Those are both great additions to any PS2 collection because they are cheap, common, and in my opinion, they still play well today. Rising Sun gets some hate, but I never understood it. I love that game.
Also, there is an HD remaster of Frontline on the PS3, but unfortunately, it's a little buggy in the later levels.