The answer to this question is determined by what is blocking you from accessing a website.
If the website itself is blocking you, then you have one set of solutions. If there is something between you and the website which is blocking you, then you have a different set of solutions.
Unblocking Websites That Are Blocking You
Most websites that block peoples access do it by blocking a specific IP address or range of IP addresses.
This type of block is designed to block where you are coming from.
The way to get around this type of block is to change your IP address, either permanently or temporarily.
Unblocking Websites That You Are Blocked From
More often, your access to some websites (such as MySpace) will be blocked by a "content filter" on your network. Content filters are popular in schools, businesses, and on some public access networks.
Content filters block access to sites which are deemed undesirable, because they contain content that people don't want you to see, or at least don't want you to see right then.
Most schools block access to MySpace, because it can distract students from their studies. Content filters typically block pornography, to keep it out of schools and workplaces. Other content which is commonly filtered includes websites about drugs, politics, religion, gambling, hacking, and entertainment.
This type of block is designed to block where you are going to, not where you are coming from. The best way to get around this type of block is to use a anonymous proxy.