I use Rescue Time https://www.rescuetime.com and the free version does the job for me.

It tracks your time spent on all sites, apps etc and gives you reports at the end of each day/week. Not only can you track your time but you can categorise your activity depending on how productive it is so at the end of the day it's very easy to work out how much of your time was spent in designer news vs actually working. You can also set goals for yourself.

Curios to see what others are using

I use fanurio http://www.fanuriotimetracking.com for time tracking. Fanurio has a few reminders to help you start, resume or stop the timer. It also has idle time detection and it can be used on Windows, Mac and Linux.

For personal use, I use a combination of RescueTime and WakaTime. For office use, we use Harvest - however, I find something like Team Snippets to be far more valuable than team time tracking in the long run for team productivity reasons. It helps everyone better understand who's working on what, why, when, what they're stuck with, etc. I don't agree with fixed hourly billing. It's a simpleton's measurement tool. No one project is the same, the needs, vision and solutions always differ.

I'm using Timings and GrandTotal and I definitely recommend it! Check it out: http://www.mediaatelier.com/Timings2/

Cheers, Rafael