There are some cool ideas in this thread! I just began ATC's, so I don't really have a collection yet but I made this holder for my mom, who has a bunch!
It's just a mini-photo album that I found at Goodwill - I removed the chunk of pages and mod podged some nice fabric (what would I do without mod podge?) onto the outside, then the inside, then glued a ribbon on the outside for closure. I used hot glue to put the pages back in the way they were. It was very easy!