I want to use a linux reverse proxy (CentOS) to serve multiple OWA sites, for multiple Exchange servers, on different domains, so I can use only one public IP address. Can I do it with apache? Or should I use Squid?
You could use mod_proxy or SQUID easily. Really, options are endless. I use SQUID for this purpose but that's because it fits better within my current infrastructure.
Mod_proxy absolutely works...Most of the proxy tags can be easily nested inside a VirtualHost directive, which makes it very easy to separate out your different domains.
If you've got a bunch of them though, I'd definitely recommend using Squid. Mod_proxy is nice, but its a proxy service bolted onto a webserver, and, as such, not as capable as a dedicated proxy like Squid.