I used to list all the challenges I could find around the internet but it was time consuming, and often times the challenges would close without me realizing it. Since that time, two groups have formed to keep all that info on hundreds of challenges in one place, and it stays routinely updated. I should say two places, because they are two independent groups and there are differences between them. You will decide which one is organized in a way that works better for you. And be kind if you ever find inaccuracies or flaws, as both are run completely by volunteer staff who take a day out of their lives every week to update the data on their list of challenges.