One example is the Logitech Trackman Marble FX. Logitech discontinued them after 2-3 years of production, and has yet to introduce a replacement that feels or works as well in my opinion.
My wife and I buy them whenever we find them available, but a shrinking supply has driven the prices up into the stratosphere – we paid $75 for one from a friend a year ago, and used prices on Amazon are almost $300 the last time I checked. I can occasionally find one for less than $10 in the bin at Goodwill Computerworks, but they’re few and far between.
I would love to see Logitech introduce a modern version of the Trackman Marble FX.
December 2008 Update: Logitech has done it again, only this time they’ve discontinued the G3 Laser Mouse.
February 2010 Update: My wife passed away suddenly in June of 2009. I’ve sold her three Marble FXes, but will leave this post open for comments.