lucky.rb outputs a link to the top hit on Google for a given search string. I wrote it to use in MacOS X with Automator + Services so that I could have a global keyboard shortcut that would replace a string with its "I'm Feeling Lucky" result-- handy for writing blog posts, comments, etc.

Usage: lucky.rb [options] query1 query2 ...
        --credentials FILE           A file containing a Google API key and referrer URL
    -f, --format FORMAT              The output format (url, html, or markdown)
    -k, --key KEY:URL                API key and referrer URL (overrides credentials file)
    -h, --help                       Display this screen


To use lucky.rb, you'll need to have an API key to use the AJAX API.
You can get one here: http://code.google.com/apis/ajaxsearch/signup.html

By default, lucky.rb looks for your credentials in ~/.google_key in the following format:
line 1: API key
line 2: the "website URL" you entered when signing up