The W3C Markup Validator is an entirely open source project hosted by W3C: there are many ways you can help make it a better, friendlier, more powerful software, and keep it free forever:
Help Others. Great communities make great tools.
It only takes: A few minutes of your time, and a bit of patience. Subscribe to the validator's mailing-list and assist people who may come to the list with questions or requests. What you get: The satisfaction of helping other and helping the project by freeing developers' time; Participation in a community of smart people who know the web really, really well.
Code. The best way you can help the project is to get involved in the software development.
It only takes: some knowledge of the Perl programming language, a few hours to get familiar with the validator codebase, and however much time you're comfortable dedicating to the project. See the Developer's Guide for info on getting started, then browse known bugs and enhancement requests or look at the architectural roadmap. What you get: A chance to get your favorite features developed, or your most hated bug fixed, in priority… plus the thanks and respect of millions of validator users.
Feedback. Your ideas matter, and we want to know about that bug you found.
It only takes: A visit to our feedback page. What you get: Much more chance of seeing that bug/feature taken care of than if you keep the idea for yourself.
Money. The validator is free. But it costs a lot to develop, support, host and maintain.