Usually, when you click on a link or type in a URL, your computer contacts your ISP's DNS servers and asks it to get the IP address of the link, so that you can make contact with it. When your ISP decides to block a certain website, it might prefer to block/poison DNS requests for the site, rather than blocking the entire IP address. In such circumstances, using alternate, uncensored or "open" DNS servers can turn out to be an easy workaround.
Open or public DNS servers are something of a rarity due to reasons of security and frequent abuse. But, nevertheless there are still a few of them around.
The most popular and reliable of the lot are the ones provided by opendns.com. They are a free (albeit commercial) DNS service that will allow you to bypass querying your ISP's DNS servers, and thereby avoid any censors placed therein. To use this service, change the DNS servers for your Internet connection to: 188.8.131.52 184.108.40.206 OpenDNS has a page detailing the steps to be followed on your operating system to point your connection to their DNS servers.
OpenDNS is one of the companies providing an alternative and free DNS service. However, as comments on this page have suggested and now, in my own personal experience, OpenDNS has started to censor a number of sites presumably to pander to their corporate clients.
Google, on the other hand, does not appear to be doing so. To use their servers, change the DNS settings for your Internet connection to:
Depending on the state of your ISP, you will very likely also notice quicker load times when you are browsing.