EC2 for some things, dedicated hardware for some others.
Where it sits, however, is only part of the problem!
I use plz[1] and instagit[2] in tandem via post-receive hooks to do the "deployment" bit, along with a service registry (closed-source, written on work time, boo!) like seaport[3] and a proxy based on httppp[4] to route requests to the right application.
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1. https://github.com/deoxxa/plz
2. https://github.com/deoxxa/instagit