Because reputation systems are needed. SPF provides authentication, but that doesn't tell you whether you're talking to an authentic friend or a very authentic spammer. Neither authentication nor reputation is any use on their own.
SPF is an authentication protocol, karmasphere is a reputation and authorization system. Karmasphere is the next required step in the chain to an accept/reject decision. It uses SPF as one of the fundamental operators in the DP Language at the core of the system.
Karmasphere is platform independent. Right now, we have client libraries available for most popular languages, all written in a platform independent manner. We need someone to contribute a windows implementation of the socket file and build files in the C distribution, but both the C and the Perl are both known to build and run on windows.
Karmasphere keeps features on the server, not in the client. It may be that the feature you are looking for is available as a server plugin or can be written as a DP program. If you think we need some extra functionality you could ask on the dev list or even dive in and code it yourself !
As soon as anyone can build and maintain them. All the developers use different distributions so we have tested on most distributions, but we do not currently maintain official packages. Volunteer packagers would be most welcome and should contact the mailing list.