Flickr, owned by Yahoo for the last 4 years, is simply the best photo sharing web service out there. Flickr defined web 2.0 with its simple, uncluttered interface, its use of tags and groups, its privacy controls and many other cool and innovative features.
iPhone and iPad apps tagged "Flickr viewers."
Exposure: A good free flickr viewer
A lot of us use flickr to store our photos online. Bought by yahoo in 2004, this service is the "gold standard" in online photo management, and is in some ways the poster child of web 2.0. Undoutably, Yahoo will eventually release a native flickr app, but for the time being we have Exposure, a free ad-suported flickr client.