Juxtapost is a Photoshop plugin for creating web collages like this, this or this.
Collages are built quickly by selecting images in Bridge and choosing amongst the matching templates suggested.
Afterwards, you can move images around, resize, delete and drop in other images. Borders can be turned on or off and both borders and background colors can be changed. When you are done, Juxtapost will let you save a JPEG file, sharpened for web publishing and with your desired quality setting.
Juxtapost comes with more than 40 different templates with different border and spacing flavors. You can generate your own templates, either by using the included Generator tool or creating your own in Photoshop.