This month, it's for the internet archive, also known as the wayback machine.
I realized recently that when I transferred my blog from wordpress to ghost in 2014 I didn't properly set the date for the older posts. And this mistake carried on until now. At first, I went to the internet archive in an attempt to recover the dates. It worked somewhat ok, but I missed a few since I rewrote my blog in ember at some point and that's invisible for crawlers :/
Ultimately I found an old wordpress backup, so now all the posts' dates are correct. In the process I realized how important the internet archive was, for the web in general. When websites come and go, it's really valuable to have a stable and sure way to get back to some information which is now lost.