There is Chris Morris' IEC and the new Watir library. Currently both
utilize the Internet Explorer "automation" interface (referred to as
COM, OLE, ActiveX, etc.) using WIN32OLE. IEC works more with form
submits, while Watir manipulates objects on a web page by sending
messages to the,. Both IEC and Watir can be downloaded from Rubyforge:
http://rubyforge.org/projects/wtr/

Watir just had a version 1.0 release last week, and plans are underway
to make it drive other web browsers than just Internet Explorer. To get
an idea of how Watir works, the first version of user guide is here:
http://wtr.rubyforge.org/watir_user_guide.html
Disclaimer: I'm a contributor to Watir.