Photovisi offers Picasa-like collage creation, online
Photovisi is a simple online service which lets you select one of a myriad of templates, throw your images, and compile them into a digital collage. The results are very similar to those of the collage-making feature built into Picasa (Create > Picture Collage), but it's an online service.
You create your collage by adding images to an online canvas (you can add multiple images at once). What's cool is that this appears to be a local operation -- meaning, as soon as you add your images they show up on the canvas, without an upload. I guess the Flash app uploads the complete images to Photovisi once you're done shuffling pictures around and selecting a background color or image.
One thing Photovisi offers over Picasa is Facebook intergration. Unfortunately, when I tried testing it I got a connection reset error -- so I have no idea what it's like.