ZoneMinder is, and will remain, free software. We are strong believers in the free/GPL software concept and am very keen to make ZoneMinder the best software of its type that you can get, whether free or paid for.
Any time spent on ZoneMinder is time that we are not doing paid work for someone else, and sometimes that's a lot of time! This includes continuing development as well as answering queries and providing support to current and prospective users. This is an 'invisible' cost but we also fork out for various bits of hardware that we think it would be useful to support and that is a very 'visible' cost as many of you speccing cameras and cards have discovered!
Therefore if you have used ZoneMinder and you find it useful, why not help to fund further development? We accept 'donations' via our bountysource page, which allows you to directly fund individual bugs, issues or features. Bugs, issues and features which have 'donations' or 'bounties' placed on them will be paid to the developer which solves that particular 'bounty'. TL;DR: You are directly funding open source developers for their work on ZoneMinder.