Opening Up a Service Through an API

I wanted to get advice on opening up a Rails application to allow an external service to make REST calls to my service. Currently, REST calls are only made securely through the out of the box Rails security. However, I would like to open all of those to external users via an API.

For example, right now people can create blog posts once creating an account and logging in. However, I would to be able to allow a third party to be able to create users and then allow blog posts for users by making API calls.

I am hoping to get an idea of what are the possibilities and what will keep it the secure as possible.

Thanks

Here is some advice for older version of Rails

http://stackoverflow.com/questions/247110/looking-for-suggestions-for-building-a-secure-rest-api-within-ruby-on-rails

Any Rails 3 advice?