I've been getting a few complaints about various sites not being accessible by Comcast users and discovered that quite a lot of domains are inexplicably not resolved by their DNS servers. Here is an extremely partial list, generated purely at random:
These are all popular blogs, and none of them work on Comcast's DNS. So I switched to OpenDNS, a free public dns service. You can sign up for an account, which gives you access to some features like porn-filtering and statistics on DNS usage. You don't have to have an account, however, you can just go ahead and start using the servers: 22.214.171.124 and 126.96.36.199.