OpenDNS is a free DNS Lookup service. You can use it to replace your ISP's DNS. OpenDNS provides an additional security service that allows you to "block" certain URLs.

Also like what *autonkoyokot said, OpenDNS can only speed up host name resolution/lookups.
A DNS lookup transforms your URL (e.g. www.google.com) into an actual IP address (e.g. 208.67.200.201) that your client uses to communicate. Once your client application gets the IP address of the host, it may no longer need to perform DNS lookups anymore.

If you find your current ISP taking a considerable amount of time in resolving host names, you may want to give OpenDNS a try.