To circumvent internet filters you must use a web proxy.
The best way to do this is to use the tor network.
tor, is free, opensource and developed by the EFF, so it's trustworthy and safe to use.
tor on its own is tricky to setup so i recommend a very easy to use package called vidalia which installs tor and other necessary apps.
this will installed tor, privoxy, and the tor button for firefox.
once installed, filter-free surfing is as simple as clicking the'Tor Enable' text button in the bottom right corner of firefox.
if you dont already surf with firefox, download it here
if you need to install the tor button then download it here