I have taken the contents of this page down because I realised that many of the challenges were sadly no longer in existence, and in fact many, many more have been created since this page was first published. If you want to find a quite good list of challenge blogs please visit Paper Playful, where you will find lists of challenges as they appear with the current challenge, also lists of winners. etc.
HOWEVER, PLEASE ALWAYS READ THE RULES before entering any challenge to make sure that you haven't exceeded any combination limit, or that you are not infringing any other rule they have in place. Also PLEASE ALWAYS READ the full challenge criteria. You have no idea how frustrating it is for busy Design Team members to visit a blog only to discover that they haven't actually read the challenge at all but just saw the title, assumed (often wrongly) that it covers their creation and thrown it into the mix a bit like a hit and run accident. Please do not just go off the title line on Paper Playful, but open up the challenge in question and have a good read.