I started using Toggl app recently and so far it's been a pleasurable experience. It takes time management to a whole new level for me. I think the best features so far are the 'Add time entries retroactively' and the 'Add tags' features. Makes time management easier.
One of the best sites if you want to start tracking your time. You create clients, projects and you start the timer. It's a great tool for procrastinators! I use the free version which is nice and quite enough for me. Easy to use, and efficient. The only problem is you need to discipline yourself so that you really use it every day, otherwise it does not make much sense.