While working on OAuth implementations for our clients at Mashery, one of the biggest issues I see developers running into is how to debug and fix invalid signature errors. There are numerous OAuth libraries out there, in fact we even have our own, so how do you determine which side is really generating the correct signature and which has a flaw in the logic? I find that using a third party library is a great way to quickly zero in on which side is at the root of the issue. The problem, however, is there are no readily available tools to do this. I have found a number of test applications, but they pretty much are for testing wether a consumer library/app is working correctly against them.
We mostly deal with providing the service provider side of OAuth, meaning numerous different endpoints, so these tools were of little help. I ended up writing a down and dirty signature generation app using the C# OAuth library (so Windows only folks). It will generate a signature based on the different OAuth parameters you enter. You can use this to compare the signature it generates to that from your consumer app or service provider. Full source code and more detailed information can be found on the Mashery Customer Solutions site. Time permitting, I may continue to add features to this tool. Hopefully others find this useful as well.