1) I am so used to a mac that makes me save lots of time compared to other systems I am not used to. That doesn't mean a mac is better or whatever, I just want to point out that using the same system for a long time is probably the best time saver there is.
2) notational velocity is the note taking application (for my mac) that I have been looking for years. I have to write something down every now and then and I can reach NV just with a key press, and get back to work. NVs ui is so intuitive and blazingly fast, that I fell in love with it instantly.
3) spotlight (as a program starter and a desktop search enginge) and time machine are IMO the most time saving things built into the OS (and the user interface, of course).