Out of the 890 UNESCO World Heritage Sites, I'll bet that the average person has seen 2 in person -- maybe 3. With Fotopedia though, you can view them all, anytime you want, on either the iPhone or iPad.
Fotopedia "brings together 20,000 photos illustrating 3,000 points of interest" within all 890 sites, with more pictures being added every day. Being a web-based app, each site is integrated with its description on the UNESCO World Heritage website, its description in Wikipedia, and a convenient link to TripAdvisor, just in case you plan on going on a vacation.
There are a few snafu's though that need to be ironed out. The navigation system past the home screen is a bit confusing, just randomly jumbling you through site after site. All of the sites are admittedly amazing, but a top level navigation menu that separated all the countries would be greatly appreciated.
Fortunately, there is a useful map view that pinpoints all the locations for you. Depending on your zoom level, it also goes from individual sites to whole countries, showing the sites within. Again, a top level view of all the countries would be much easier, but such a menu does not exist.
For a free app, Fotopedia has quite a lot of content, making it a borderline "must have." It could use some organization, but if you want to go on a picture tour, this is definitely the way to do it.