Ok I was smart this time, did it in Notepad! Here is a long list of sites that have challenges, I will add this to the Digital FAQ too. I'm sure there are tons more sites out there with challenges, but this should definately get you started. I normally do all the SOTB & Digitals challenges (I have CT duties at both sites, so I like to help promote the sites, plus the mega kits of bonuses is awesome!), I do a few at SBE & I do all 5 at DDD.
Scrap Outside the Box - has 19 challenges each month, the posting bonuses all coordinate with each other. One is a posting bonus that you need to sign up for, so do that one quickly! If you complete 12 challenges, you get a few bonus QPs.
Digitals - has 17 monthly challenges, with coordinating posting bonuses. Many of them have a few pieces that you need to download and use in the challenge, but you have to post that you want the pieces and it will be PMd to you, so do that soon. If you complete 12 challenges, you get another bonus kit.
Gotta Pixel - I've never done them.
Digital Divas Design (DDD) - has 5 monthly challenges. Each challenge you complete increases a percent off coupon in the store, 1 challenge is 10%, 5 is 50%. If you post layouts there, you have a decent chance of winning their layout of the day contest, which gives you a .50 cent gift certificate for each win and you can combine both during the next month. If you win LOTD 3 times, you get a gc for $1.50, so I look for items in the store for $3.00, use the 50% off and then that makes it $1.50 and use my gc so there is no cost.
Divine Digital - has a few weekly and monthly challenges with posting bonuses. They also have a daily challenge series with a theme, that gives freebies each day.
Scrapbook-Elements - has a few monthly and weekly challenges with posting bonuses.
The Scrapping Garden - has several challenges, I have never done them.
Digital Scrapbook Place (DSP) - has many challenges, some monthly, some weekly, some on weird schedules. Some have posting bonuses, some have random winners of kits, some have best LO wins a kit. They have a couple challenges with no bonus, so read carefully before you do them.
Digital Freebies - has challenges, but their site is down now, so I can't check them out, I have not done theirs before.
3 Scrapateers - has several challenges, I have never done them.
Go Digital Scrapbooking (GDS) - has several challenges, plus it's their 1st birthday til Sept 8th, so they have extra challenges and games and free stuff now til then.
Scrapbook Bytes - has several challenges, I've never done them.
I imagine that's quite enough to get you started!!