Firstly check this link below from CNET if you have the same problems.
http://reviews.cnet.com/8301-13727_...ed-web-sites-after-os-x-update/?tag=txt;title
Check your internet billings. Or check with your ISP for any network outage.
Check the settings of your Wifi Router. Or connect directly to DSL or Cable modem. See if you can surf the web.
Isolate one by one.
If you have a windows pc. Goto thru the steps which I mentioned earlier.
On your Mac.
Go to terminal and check your /etc/resolv.conf
Type this command sudo cat /etc/resolv.conf in your Terminal. While connecting directly to your DSL or Cable modem. Not your Wifi router.
If you can surf the web from connecting to the DSL or Cable Modem. Then the problem must be the settings on your wifi router.
See what DNS is printed there. (Check the steps for your Mac option from my previous posting )
Call to check with your ISP whether you got the correct DNS settings.
From the terminal you can flush your DNS cache. By typing this on the terminal screen sudo dscacheutil -flushcache
In Lion and Mountain Lion
sudo killall -HUP mDNSResponder
In Leopard / Snow Leopard
sudo dscacheutil -flushcache
In Tiger
lookupd -flushcache
If you know Unix commands such as ping, traceroute, nslookup, dig. That will help a bit. Do a ping and traceroute on your Mac terminal screen.
There is DNS Changer Malware for the Mac too.
check the steps below:-
http://www.dcwg.org/detect/checking-osx-for-infections/
(goto the link which I posted before this post under us-cert.gov DNS Changer Malware.
http://www.dcwg.org/fix/)
Make sure that your Mac OS X root account is disable.
"Wake Up!" Rage Against the Machine