My favourite keyboard of all is the Matias Tactile 3 Keyboard from Canada.  It has mechanical switches and makes a lovely clickety clack sound.  Good tactile feel and the keys are laser etched with the alternative keys.  It isn't cheap at $150, but IMHO it is the best.

Here is a review from David Sparks.

http://www.macsparky.com/blog/2010/11/15/review-matias-tactile-3-keyboard.html