They host pretty much everything -- Maps, Gmail, Search, etc. Highly suggest reading the Wired link below as it talks about this in detail [1], but if you want a more technical look, check out the Youtube video [2], or the Borg paper [3]. One really interesting development, is that Mesos + Marathon + Chronos + Docker, puts a very similar system into the hands of your average IT team today [4].

[1] http://www.wired.com/2013/03/google-borg-twitter-mesos/

[2] https://www.youtube.com/watch?v=0ZFMlO98Jkc

[3] https://research.google.com/pubs/archive/43438.pdf

[4] https://www.youtube.com/watch?v=hZNGST2vIds

- crawlers

- websearch

- google brain

- video transcoding for youtube

- appengine (think snapchat)

- gmail

Almost everything Google does runs on Borg.

When I ran Exacycle http://googleresearch.blogspot.com/2013/12/groundbreaking-si... the worker pool was just a low-priority (priority 0 in the paper) job like everything else, soaking up idle cycles. The command and control jobs were just regular borg jobs that managed the worker pools by assigning tasks to them.

So whatever else was going on, protein folding and drug docking and telescope simulation was soaking up whatever leftover cycles there were.