I use plain old Notepad on Windows. "F5" inserts the date and time which I then edit a bit. I wrote a program to total the hours and such.

I started keeping a log years ago because sometimes it seems like I can't remember what I had for lunch. :-) It a lifesaver when your boss wants to know what you did last week.

Keeping a daily log comes in handy. A former colleague needed to know the details of a project I worked on a few years ago. I had him search my old logs and the answer was there (weird complier switch for using QT).