This UK site has some public galleries:
http://www.photobox.co.uk/public
Here's another, which allows you to search for photos by keywords:
http://www.fotopic.net/
(Note that to search for individual pictures you should click 'photos' on the page which shows the initial results of your search)
Picasaweb is an international site, where you can search for images by keywords:
http://picasaweb.google.co.uk/
Flickr is also international. It allows you to search for photos by 'tags':
http://www.flickr.com/
Atpic (another international site) allows searching by keywords:
http://atpic.com/
DotPhoto (US/International) allows you to browse public galleries by category. (See the links at the bottom of the page):
http://www.dotphoto.com/Gallery.asp
Fotki (US/International) allows you to search for photos by keyword:
http://www.fotki.com/
SmugMug is similar:
http://www.smugmug.com/
. . . as is Fototime:
http://www.fototime.com/
(The search facility doesn't always seem to work on this site)
Personally, I enjoy trying to work out how to use photo gallery sites from non-English speaking countries. (e.g. Netherlands, Bulgaria, Russia, Hungary, etc). I