Flickr

Wow! There sure are some talented photographers out there! I could spend hours looking at pictures.
As for mashups....Retrievr is pretty cool. You create a sketch of anything you want and it matches it with images on Flickr. I wouldn't say it works well, but it is fun to play with.
Trippermap is a lot of fun (and more functional). You can put a world map on your blog or website and if your photos are tagged with place names they'll show up on the map. Check out Last Known Location to see how it works.