Depending on the type of site that you wish to view you do have multiple options.
For any static site, i.e. you're looking for data/stories/etc, then a simple google cache should work just fine as there is no dynamic transmission required.
For dynamic sites (youtube and facebook both fall into this category) then you have one possible solution and one possible loop hole depending on the intellect on your site blocker.
a) A solution is, as some have said, to proxy to the site or even visit it using a terminal although this is not recommend because it *could* be seen as foul play and earn you a disciplinary.
b) Now this depends on the site blocker however I have found that a lot of them do block common sites like
www.facebook.com and
www.youtube.com. However, and luckily, these blockers were written in the US =D and therefore only block the US site. I have been lucky enough to simply type
http://uk.youtube.com and get straight there because the blocker will not register that as the same as the US site. Hope that makes sense and that you don't get in trouble should you try it.