While I don't charge for my work on open source projects -- I do need to pay my bills at the end of the month, and this means money.
On top of this, some of my projects (like FaceBrick) actually cost me money, as I need to run a server to host some of the code that talks to Facebook.
You aren't obliged to donate, but if you do, I'd be very greatful, as you'll be helping me to spend more of my time working on things that you and I both love to use.
You won't get anything in return except for a warm fuzzy feeling, though if you contact me, I might be willing to look into something for you.