Open Source Engineering Software?

pipingguy asks: "A long-time subscriber to my piping design/engineering-related discussion list inquired about Open Source engineering software (and expressed an interest in moving to Linux), citing the high cost and often poor customer support of the commercial packages. Since there are quite a few non-Software Engineers reading Slashdot, I thought I'd ask a 'somewhat larger' userbase by posting the question here. What Open Source engineering software is out there and useful?"