Recs.
Updated
Toggl is a web app for the Chrome browser (with native apps available for Windows, Mac, iOS and Android) that can be used on Linux.
SpecsUpdate
Pros
Pro Active time tracking approach
Someone tracks their time to extract value from that: usually looking to increase productivity. And I only archived that with the active time tracking (versus the passive automated way).
In the active way you must manually click the timer to track the time. More than that: you must provide - again, in advance - what are you going to do. You are committed to it in advance. It's an active process which, being that, has repercussion in your time approach.
Here is the problem with an automated system: they get the data. Nothing else. You are not check it later and reflect about it after the first couple of days. I don't know about you but I have been there: weeks, months of data. Zero reflection. No value extracted.
Obs.: Toggl does have automated tracking resources as well. I don't use them.
Pro Integration with existing services
Integrates with services such as TeamWeek, Pivotal Tracker, Github, Asana, Unfuddle, Gitlab, Trello, Worksection, Redbooth, Podio, Basecamp, JIRA, Producteev, Bitbucket, Stifer, Google Docs, Redmine, YouTrack, CapsuleCRM, Xero, Zendesk, Any.do, Todoist, Trac, Wunderlist, Toodledo, Teamwork.com, Google Mail, Taiga, HabitRPG, Axosoft, Countersoft Gemini, Drupal.org, Esa, Help Scout, Flow, Sprintly, Google Calendar & TestRail.