ClamAV is an absolutely wonderful tool, and one many just take for granted. Virus definitions "magically" come from ... somewhere ... and the mail / proxy server helps secure the network border.
It's often used indirectly via amavisd-new or similar, so people may not even be aware they're using it.
Personally, I'm extremely grateful that such a tool exists, and very thankful to all those who've helped maintain definitions, write and enhance the engine, build tools, manage the mirrors, host mirrors, maintain the website, and all the endless other work involved. It's a life-saver, as it works so smoothly and with such minimal setup that I never really need to worry about the virus scanning done on the mail server. It "just works".
The only way I find myself really able to give back is by turning on stats reporting. You should too, it's a trivial one-off setup using OpenID credentials you're certain to already have. Setup took me about one minute.