If you use firefox:
Install TOR:
http://www.torproject.orgIt re-routes your traffic through a series of random servers on the internet, all of which cannot see where the final payload is going. That includes your employer. It also encrypts the data, so no-one can sneak a peak.
Then Install Torbutton add-on for firefox:
https://addons.mozilla.org/en-US/firefox/addon/2275This allows you to click a button in the bottom of your browser to enable secure traffic. It's slower than internet normally would be, obviously, and your other tabs (like gmail) will become non-usable while the browser is in secure mode. BUT: I've found it quite effective in hopping employer firewalls.
You can use this with other browsers too, but the setup and use is a bit more involved.