I use Project Hamster from the gnome project (I'm a linux user). It's free, integrates nicely with my desktop and is always available (unlike web based time trackers). It uses a sqlite database which is a nice plus (I also synchronize the database file over dropbox so I can share the time tracking on my other machines).
At the start of a new month I export the time reports for the previous month from hamster as XML which I then import to my custom billing engine (I tag each task in Hamster by client name). This then gets converted to a time breakdown on each client invoice. Clients can then view a detailed breakdown of the billed work at per minute granularity along with descriptions of each subtask.
At the start of a new month I export the time reports for the previous month from hamster as XML which I then import to my custom billing engine (I tag each task in Hamster by client name). This then gets converted to a time breakdown on each client invoice. Clients can then view a detailed breakdown of the billed work at per minute granularity along with descriptions of each subtask.