I'm looking for a library that will allow me to use OAuth in my ASP.NET/C# applications, such that I can authenticate users using one of the following OAuth providers
Jackie I have a blog post on this OAuth C# Library
There is a library (including source code) and a simple sample project you can download to get started with this.
I've tested it against 1. Google 2. Twitter 3. Yahoo 4. Vimeo
You don't mention the version of OAuth you'd like to support so you should know that the library supports OAuth 1.0 revision A only and not OAuth 2.0. Most site today support OAuth 1.0 revision a.
I've kept the sample project very simple intentionally, so those starting out down this path don't have to struggle trying to figure out how to use the library. The library is not "over engineered" like I believe some libraries out there are so it is fairly simple to enhance if need be. But I suggest you attempt that only after you've familiarized yourself with the OAuth protocol.
The project in fact is an open source project hosted on Google code (link in the blog post).
Note: Due to the way Yahoo! has implemented their service it is not possible (or not simple) to test against their service from your development machine.
I hope this helps.