Since I put every interesting thing I do (be that code, documentation, poetry, blog entries, or just random stuff, including pictures I take) in git, one could count that as a kind of diary.

I've been keeping such a thing for over a decade now, first in CVS, then in TLA (aka GNU Arch) and now in Git.

All my mails since about 1998 is kept and archived aswell.. and the reason I keep everything, and store them in git, is because I have an unnatural desire to cling on to every bit of stuff I wrote, every discussion I've been involved in, and so on. Having it in git also allows me to easily replicate all or parts of it on my different machines, as a cheap backup solution.