World of Padman is a delightful freeware arena shooter with an impressive amount of work put into it! It's meant more for multiplayer, but the bots are pretty fun to play against too!
Posted about World of Padman before, but felt like posting about it again, this time with a video of gameplay.
If you like lighthearted games and arena shooters, it's well worth giving a look. Also I love the attention to detail on the first map in this video, putting jump pads on the mouse traps is so good.