The "Make Collage" feature in Picasa is freakin' great. I made a collage of like 500 photos and framed it for our anniversary.

but..

there are more tricks involved. Once you assemble the collage, but before you save it-

you can move the photos around inside the collage..via drag and drop..and resize them and re-orient them too, ie change the angle.

I did not know that!

You never used to be able to do that, I believe it was added in the 3.0 release