I have a site hosted by apache on port 80. I also have a python web server listening on port 8880, which needed accessed via http://[mydomainname]/something. Using txyoji's answer, I got it working by simply adding a proxy pass to my virtual host definition like so:
ProxyPass /something http://mydomainname:8880/something
ProxyPassReverse /something http://mydomainname:8880/something
UPDATE
Depending your setup, an even better way to do this is to setup a proxy pass for a port on "localhost". I think it's a bit more clear what you're doing, plus more portable. Along with this, you don't have to even open up the firewall to that port! You can proxy pass locally to any port, so there is no reason to expose that to the outside world if you don't have to. Funnel everything through port 80 and have Apache always "run out in front". Then, you can just worry about the security of that.
ProxyPass /something http://localhost:8880/something
ProxyPassReverse /something http://localhost:8880/something