A number of forum posts have been asking why OpenTK doesn't have a prominent "donate" button, and I noticed that there is a Paypal-based one at the SourceForge project site.
If you've found OpenTK as useful and high-quality as I have for your .NET/Mono apps, show your appreciation with a donation! If you're a developer using OpenTK in a commercial application, talk to your boss about having your organization make a donation.
As it says on the donation page:
OpenTK is free to use but costs real money to develop and support. If you feel this project is worth it, consider donating a small amount to help it along. Every little bit counts!