Different challenges start on different days of the week and run for different periods, so always check out the blog for details. Some even have prizes at times, but they are all fun. I find them great for inspiration either by participating or just purely by looking at what other people have done.
The list does not cover all the challenges that are out there, as there are hundreds, but it will certainly give you a good start. If you happen to discover one that you think is extra special, then feel free to send me an email and let me know. Its always fun to check out new things.
A list of challenge blogs (in no particular order).
Mojo Monday - http://mojomonday.blogspot.com.au
Freshly Made Sketches - http://freshlymadesketches.blogspot.com.au
Global Design Project- http://www.global-design-project.com/
Stamps, Ink, Paper - http://stampinkpaper.com
The Paper Players - http://thepaperplayers.blogspot.com.au
Just Add Ink - http://just-add-ink.blogspot.com.au
Pals Paper Arts - http://www.palspaperarts.com
Addicted to CAS - http://addictedtocas.blogspot.com.au
Fab Friday - http://fabfridaystampinchallenge.blogspot.com.au
Sweet Sunday Sketch - http://sweetsundaysketch.blogspot.com.au
Merry Monday - http://merrymondaychristmaschallenge.blogspot.com/
TGIF Challenge - http://tgifchallenges.blogspot.com.au