This tool is donation-supported. That means that we do not track users with commercial purposes, sell advertisements or offer any paid services. However, we rely on your donations to fund software development, the purchase of books and software, and cover our infrastructure costs.
You can make a donation of the amount you want by using PayPal or Bitcoin. If you're looking to make a different kind of contribution, please think about contributing to unrelated organizations such as MSF or Save the Children.