Delivr combines the power of Flickr and Creative Commons to create what may be the largest single repository of online postcards. So, if you're looking to share the wonder of the Field Museum, the vistas of the Sears Tower, the sheen of Millennium Park, the grittiness of the CTA, or a picture of just about any other keyword you can imagine, you're in luck. (NB: To use your own photos, you'll want to make sure that your CC license setting -- either at the default or individual image level -- is one of those that allows derivatives.)