I will probably sound stupid and get a collection of DUH's but I figured I'd mention it anyway :

Do not let any engine degreaser or any oil dissolving chemical get on your timing belt! If you do, don't drive until you changed the belt!

I was looking for a leak on the front of my engine (see post "help pinpointing engine oil leak please"). To get an idea where the oil is coming from I used some Oreilly brand engine degreaser. It cleaned everything nicely but I still could not see the leak since it seemed to come from under the timing belt cover. I took the cover off and ran the engine to see if I could make out a source. (I didn't use any more degreaser at that point) Thats when I noticed the timing belt had absolutely no tension and I am lucky it didn't jump or break. I have driven the car to work and back 50 miles the same day so I doubt the belt was like that the whole time, I would not have an engine by now. The only thing I can imagine is that the degreaser, which is made to dissolve oil, had gotten onto the timing belt (made of rubber which is made from oil) and softened it so much that running the engine stretched it to where it had no tension anymore.
I am now replacing the belt and while I'm at it, everything else hard to reach around there that needs to be replaced or can leak (waterpump, cam seals etc.)