I'd say Borderlands 1 and 2. They aren't actually similar really at all, but there's an addictive speed to the running around and the shooting that's much like Halo.
Otherwise, and this isn't exactly like Halo, but the Half-Life 2 series has a similar ease of shooting, some interesting environment puzzles (some tactical as in Halo where you figure out how best to use your surroundings), and it's very cinematic. I think Halo was one of the first games to mix the cinematic with shooting in such a way. Half-Life 2 is similar in that respect. Plus it's just a really cool story and a blast to play. The original Half-Life game is fun too, but it's not at all similar.