Every morning I have to present the tasks I have been working on in a meeting called Scrum. This is helpful for a number of reasons. It keeps everyone focused, the team knows what is going on and who to talk to about specific issues, keeps people from working on the same problem, etc..
One thing that bugs me though is that I can never remember the full list of things I have been working on. To fix this I started making a list of my daily activities in a file called WhatIDid on my Desktop. This actually worked really well.
Then recently I needed to give my boss a list of all the project I had been working on recently. This was really hard to do as I have done a ton of work on about every part of our projects architecure. Thats when I decided I would use Github as my diary.
I now commit my WhatIDid file in a private repository with the date as the commit message. This way I can browse back through time whenever I want and see all the work I have accomplished when necessary.