Google has made available two free accessible name servers. ISP nameservers have often difficult to remember IP addresses, and sometimes do not work all the time. Google has started proving a name server service, the following addresses work: nameserver 8.8.8.8 nameserver 8.8.4.4
(For Linux users, enter this in /etc/resolv.conf if you do not run a 'named' locally)
This also implies that google controls where you go if you type the name of some site... But that may actually be an advantage if you live in a place like for example China that tries to kill the freedom in the internet.
Can also be handy to test your net connectivity; ping 8.8.8.8 is easy to remember.
Maye be a bit slower then your ISP's if it is further away.