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.
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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.
Pro Open source, native Linux desktop client is being worked on
Con Stable version requires Chrome
The stable version of Toggl on Linux installs as a Chrome app thus Chrome has to be installed on the machine and run (it can be run as a process in the background). A native client is in the works, but it's still in beta.