Openvpn supports operation over an http proxy. Its client is available on most desktop platforms; the server is available on most unixes. Pro: you can use any network protocol transparently (including UDP and streaming). Con: some setup required; the overhead is noticeable in some applications; you need admin rights on both sides.
Httptunnel and Httptunnel each let you set up one TCP connection (e.g., ssh) over HTTP. Pro: you don't need admin rights on either side. Con: you need special software on the server side; expect noticeable overhead; ease of setup and use can vary.
If you're only interested in ssh, you can tell it to operate via the proxy by using a program such as corkscrew. Pro: little setup required; no special server-side setup; no connection overhead. Con: doesn't work if the proxy is too restrictive (e.g. doesn't let https through); specific to ssh.
It may help to run the server on port 80 or 443.