"Often when creating an article or a blog post, we want to generate useful keywords that search engines will index on. This can be done directly from the content itself using the Yahoo Term Extraction API.
The API is extremely easy to use: you pass in a string containing the content as a POST variable, along with your API key, and you get back a string array of terms that Yahoo recommends.
Learn about the Term Extraction API:
Get an API Key here
Here is a class I put together that handles everything:
Is it sad that the first thing I thought about when reading this that it would make a great Windows Live Writer plugin?