CPANdeps can now filter results by OS. Because people asked for it. Personally, I don't think it's very useful. The purpose of the site was originally to help module authors determine which modules are good to depend on because they're easy for your users to install. It's rare for a module author to have control over what OS his users are suffering from (the obvious exception being platform-specific modules like Solaris::Procfs, BSD::Resource, or Win32::DriveInfo) and so filtering by OS isn't helpful.
