Klok (All platforms with Adobe AIR, Free)
Built with Adobe AIR, Klok is a lightweight and cross-platform tracking solution. You can create a hierarchy of projects and sub-projects in the task-management sidebar and then track the time spent on each by dragging and dropping them into the workflow for the day. While you can delve into the details of each block of time, simple adjustments like expanding the amount of time you've worked on a project is as easy as grabbing the edge of the block with your mouse and tugging it down.
Manic Time (Windows, Free)
One area of resistance many people have to using time tracking software is a fear that they'll waste too much time actually plugging information into the system. Manic Time alleviates that fear by actively tracking what you're doing on your computer to make tagging and analyzing your daily work flow simple. Your day is represented by three time lines: Activity (either on the computer or off), Applications (which were open), and Tags (your personal annotations to your work flow). Mousing over any of the three time lines gives you additional data about that moment on the time line and you can always pull up the statistics window to see your work patterns over time. Check out their video tutorials to see Manic Time in action.SlimTimer (Web-Based, Free)
SlimTimer is a web-based tracking solution. Once you've signed up for a free account, you can begin creating new tasks you want to track. You track those tasks by flagging time you spend on them in the little pop-out time manager you see in the screenshot above or by keeping the SlimTimer web site open. You can add tags to your tasks in the management section of the SlimTimer site as well as share tasks with coworkers. If you're concerned about using a web-based tracker and losing control of or flat out losing your data, you can export your time-tracking data or even have SlimTimer email you a backup once a week.
RescueTime (Windows/Mac, Free)
RescueTime aims to be the least intrusive time-tracker you'll use. Rather than have you log each individual activity you do in a journal-style system, RescueTime monitors the web sites you visit and the applications you use. You can set goals in RescueTime based on a variety of factors, like how much time you want to spend doing certain tasks or how much time you want to dedicate to certain projects. RescueTime analyzes your computer usage and reports back to you on whether or not you're meeting those goals. RescueTime takes a different approach from most time-tracking tools, so we'd definitely recommend reading their FAQ file for additional insight into how they handle time tracking.
Project Hamster (Linux, Free)
Hamster is a simple time-tracking tool for Linux-based systems. You enter tasks as they occur, and then you can categorize and sort them. When you're done with a given task, you simply tell Hamster to stop tracking it. You can also set up a reminder system that will remind you to record what your current activity is every X number of minutes. One of the more interesting features of Hamster is the ability to shift how your day is defined. If you're a night owl and do most of your work after hours, Hamster won't slice your "day" in half at midnight just because the clock says it's a new day.