This walkthrough demonstrates how to create a Ruby Gem from scratch to interface with any API. The author is Ben Lewis writing for Engine Yard's blog. Referencing an imaginary web-accessible API called "bens benzes," the tutorial uses bundler to piece together the gem step by step. Users can clone the resulting gem from a GitHub project accessible from this page.
Mashup contests can be a good way to reveal the variety of applications that are possible when the developer community is provided with a quality API. At the recent BarCamp London 5, Stefan Magdalinski, CTO of MOO.com, ran a weekend-long MOO API hack competition. The MOO service and API let users submit custom designs that can be printed onto products like business cards, sticker books, postcards, etc. These products can then be sold through an online store.