One of my favorite sites is Scrapblog. Scrapblog is basically an online scrapbooking site. It's a fun (and addicting) site that has lots of free items, as well as many that are available for purchase. It does require that you create an account but it's free and simple to do. Click here to take a quick tour of Scrapblog and see how it works!
Another of my favorite collage sites is Picnik. Picnik is a fabulous online photo editor that also allows you to create some basic collages. I use it all the time to edit my photos- from school and from home, as well as to make collages. It's very simple to learn and many of the features are free and don't even require you to login to use them.
A site that I just recently found to create a picture collage is Photovisi. Photovisi is a free, online collage creator that is simple to use, allows you to customize, and requires no login at all!
Here's a collage I created in just a few short minutes this morning