How do I get rid of browser redirect malware? I am using Mac OS X Leopard on a MacBook Pro.

I must have been very careless at some point, since I managed to let some malware install in it. 

While browsing I get randomly redirected to different web sites than those I intended to visit, both from Safari and from Google Chrome.

I have tried to clean my computer with ClamXav without any success so far.
Sounds like your DNS might be hijacked, which could be your router or your Mac.

I'd suggest using OpenDNS for a while to see if the behaviour stops.  If it does, then you will need to look at getting your DNS reconfigured.

If the behaviour continues then it may be malware but honestly I can't think of a single one that would do this.