open source anonymizing web proxy

a friend of mine had asked me to look into this. she is frustrated by
the fact that her employer blocks 90% of the websites that she wants to
access during the day from work. is there an open source web proxy out
there that anyone knows about that i can setup on my linux server that
will help her out? the problem with using commercial solutions such as
anonymizer.com is that her company blocks all of those URLs so the only
way for us to get this to work is if i set up an anonymizing proxy on
my linux server and she can then surf the web via my proxy.

thanks for any input.

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