FlickrNet is a .NET library for implementing all of the Flickr API's methods. It is designed to reduce the complexity of calling the API, especially with regards to authentication. This library can be accessed from a variety of frameworks and is written entirely in C#.
The Aviary Effects API that helps developers create their own Instagram recently made the API public and updated its developer portal. In addition, the company is offering a short-term developer contest, so you'll need to get going quickly.
While map mashups are what have defined this genre of application, the second most popular type of web mashup here on ProgrammableWeb are photo mashups. How popular? Just this past week the number of photo-related mashups passed the 500 mark, and there are now 505 listed.