In Munchkin-funny games, the components are funny. The cards have funny names or flavortext, and it's amusing to be attacked by thousands of orcs while you have a duck stuck to your head. I'd put Illuminati and Chez Geek in this category as well (not that Steve Jackson Games has a monopoly on these). These games are very funny to begin with, but (to me at least) become less amusing as you become more familiar with the cards. It's probably no coincidence that Munchkin and Chez Geek have a lot of expansion sets.
In RoboRally-funny games, the gameplay is funny. You make plans, you have an expectation of what will happen - and then something completely different actually occurs. Instead of sprinting along the conveyor belt and jumping off just as you reach the flag area, someone accidentally pushes you onto a turning block and you sprint in the entire wrong direction, jumping onto the conveyor belt that throws you into a pit. I'd also put Wiz-War and maybe Fluxx in this category. These games don't sound as funny on first glance or on a read-through but in actual play both the players and bystanders were laughing raucously as our Galaxy Trucker ships got blown to pieces by asteroids and pirates. These games stay funny as you play them.
A funny subject or cards, like in Munchkin, can be applied to a very strategic game (I'm sure there's some way to make Go funny) but RoboRally-funny games are by definition not strategic at all. I'm sure some people would be too frustrated by this to enjoy the game but I really like them.