Instant is a menu-bar app that tracks the amount of time spent on your Mac everyday! It also tracks the number of standbys and lets you set limits to your daily usage as a notification. You can even export the data. It's a must-have app for all Lifelogging and Quantified Self enthusiasts. Instant can help you keep your Mac usage in check, and can be used as an effective parental-control tool. Instant is from the award winning startup, Emberify!
Note: Now requires OS X 10.8 or later