(Disclosure: I work for Applied Wave Research. I understand that commercial posts are generally discouraged on discussion forums, so I'll keep this one short.)
Among MMIC designers, Microwave Office from Applied Wave Research has become very popular in the last few years. I work for them now, but until February I was a designer, and used Microwave Office for several successful projects.
At
http://www.appwave.com you can download a 30 day demo - it runs on Windows, but we'll have a Linux version out next year. Also this year we are introducing Analog Office, a seperate product specialized for RFIC design.
If Ken says it's ok, I'd like to describe our product further. Otherwise, you can check out our site, or email for more information.
Duncan (duncan@mwoffice.com)