I know of gems like Mechanize which let you open a website, give credentials and eg scrape the contents. I also know of AutoIt and its use through Ruby to interact with already opened windows but i wonder if there is something better and more Ruby-like like Mechanize but for an already opened and visible site, it would simplify the logon procedure which for some sites isn't easy to do in the gems i know of. I intend to use it for semi-automating frequent tasks with sites that are difficult to log on to with a bot and also to have more visual interaction. I tried it with Sikuli but although promising it isn't reliable yet.
You're looking for Watir. It automates a browser from ruby.