The largest one, with over 400 members, is The Gathering Room. You can find their button in my Left Sidebar.
This is a knit-and-crochet exchange group. When you join, you'll be invited, once a month, near the end of the month, to join one of the next month's exchanges.
The Gathering Room has a wide variety of exchanges: Angel, Square (6" and 12"), Bookmark, Pattern Book, Skein of Yarn, Towel Topper, Dishcloth, Coaster, Doily, Fridgie (Magnet), and many more, including various holiday and charity exchanges. You are bound to find one to suit your taste and skill level.
Suppose you join the Angel exchange for April. You have an April deadline to complete either a knit or crochet angel for your partner, and your partner does the same for you. It is a lot of fun to see what your partner sends.
The Gathering Room also includes great lists of links for free patterns and craft sites, some special techniques, lots of craft-chat back and forth on the Group. Members often ask about confusing pattern instructions and other knit and crochet issues.
I have a personal love affair with the photo on their Home Page. It's a big ball of scrap yarn and I just love it. It just makes me want to grab my hook and get going!
This is a warm, friendly needlecraft group and one of my faves.