Usually time tracking is considered so dull that it's one of those 'will only develop for money' application. While some people have scratched that itch, a lot of the applications a simple Google search will turn up are pay-only.
I'm assuming you already did your homework and want personal opinions.
Some people use pen and paper. Others abuse Excel to make a time-tracking database. I used to use a tool called
ConsultComm. It was developed to be an IM program, but the time tracking features were so well received that the IM part of the program was dropped. Feature creep and scope creep happen even in F/OSS. Plus I like something small that can sit in my taskbar.
If you are looking for professional solutions like
Timeconsultant, there are a lot of web-based calendars.
For independent developers
Rachota covers most the basics. I still use ConsultComm out of habit, but this one is quite nice.
The official solution at my workplace is a stopwatch built into the fat-client UI for our chosen ticketing system,
Remedy Action Request System. But Remedy is a 500lbs gorilla of the change-control, helpdesk-requests and bug-tracking markets so the little stopwatch goes unused most the time.
I think you will find that hourly clients, consulting and small businesses will be in the mindset of needing time trackers. Larger companies and salaried positions will trust you to report your time on some paper form or (more likely) at a website-based timesheet application.
Don't be surprised if you track your time honestly and find quite a lot of it wasted. That's why systems like
GTD exist. And GTD includes a lot of nifty software like time trackers and wikis-on-a-stick to support it.
In the end though, you almost always have to press some start and end button.