As Ms. Mittens stated:

The posts by instronics and raghuveer both highlight important points. If you need to get something from those sites you listed (astlavista, etc.) why not talk with the admin and explain why you need those tools -- I am assuming you are interested in them for a legal reason, i.e., to test the network. If you are determined to use those tools and he won't give access, build a network at home to play with. That's what a lot of members do -- myself included.
I would also presume you wish to get to these sites for security reasons. Our own company allows a person to e-mail the proxymaster if there is a site that is currently blocked and you need access to it. If that is agreeable then bada-bing, you're in. Keep it on the legal side. I have seen people walked out of our own company for doing otherwise. People get kinda hinky about that type of thing.