TopTracker provides a straight-forward, free time tracking utility that works on both Windows and OS X. The utility allows commenting on each session tracked (e.g., words written during that session). It also allows export via CSV, from where numbers can be crunched further in Excel to see how well progress is going.
By default, TopTracker will upload screenshots periodically while it’s running, but this feature can be disabled and other elements customized in the program’s settings.
For other similar utilities, see the Zapier blog. For additional discussion of the value of tracking writing progress or other “deep work,” see Paul Silvia’s book How to Write a Lot or Cal Newport’s Deep Work.