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