The FamilySearch C# SDK by FamilySearch communicates with the API via NuGet. It provides usage and example available at https://github.com/FamilySearch/gedcomx-csharp/tree/master/FamilySearch.Api#examples
What could you do if you were spidering two billion web pages per day? Whatever your answer is to that hypothetical, you might as well do it now. The 80legs platform lets you write your own web crawler and sell your apps to users (our 80legs API profile). And if you can build it fast, there's an 80legs contest looking for your entry.
Developers are often finding themselves as API providers in addition to consumers. The same is now true for me with my project, BuildaSearch. I recently released a BuildaSearch API for deeper site search integration, moving away from a more plug-and-play approach I took earlier on.