Google Mail Chrome Extension — TimeCamp
Start TimeCamp timers from Gmail threads and compose windows in Chrome.
Free
Start Timers on Google Mail Directly in Chrome — TimeCamp Extension
Track time in Gmail directly from the TimeCamp Chrome extension with one-click timer buttons. The TimeCamp Chrome extension adds a start button in Gmail threads and compose windows, so you can log the time you spend on client emails and internal correspondence.
Free with the TimeCamp Chrome extension.