It's a public, GPL project https://github.com/synergy/synergy You can either compile it yourself or you can go through the very smooth purchase process to get a lifetime subscription to pre-compiled binary installers for $10. It was much cheaper for me to just type in my CC# and move on with enjoying the program than to even investigate compiling it myself.
Both Synergy and Humble Bundle do the same thing: They make going through the checkout line a smooth, easy, natural process on the path to getting the software. The problem with donations is not really that people are so cheap. It's that they're so lazy. If you give them what they want up front and then ask that they come back later to go through some awkward process to give you money for something they already have... Well, what do you expect? It's too late. They've moved on with their day. They can't be bothered. But, if you make it very easy to give you money on the path to giving them the goods. Then their appreciation and generosity doesn't require much effort. That's the key.