After installing Snow Leopard I found that Safari AdBlock doesn't work when Safari runs in 64-bit mode and I had to look for another ad blocker.

And I found GlimmerBlocker: GlimmerBlocker

Even better than Safari Ad block as
GlimmerBlocker is implemented as an http proxy, so the stability of Safari isn't compromised because it doesn't use any hacks. It is even compatible with all other browsers and other native Mac OS X applications which uses http, e.g. NetNewsWire.
Enjoy