Cherry membranes, especially the Cymotion range, have a very long life spec. Also some Acer Accufeel, Benq keyboards, the ones with 6511 in their model number, are very cheap, but are rated for 20 million keystrokes. They can be found under a whole bunch of brand names. HP, Mitsubishi, etc.. Just google
keyboard 6511.
I have
this one. It's 9 years old, metal plate bottom and fairly slim for a full-travel membrane. Still rattling away, but may have been quieter when new. It was my home keyboard for about 6 years. At the time my work keyboard was Keytronic ergo-something-or-other. The Keytronic felt superior.
Anyway, my favourite membrane keyboard right now is the Cherry Cymotion line. It feels great to type on, though it's pretty loud. Then the Keytronic (the old-skool quality ergo-something), though to be honest I haven't used one in ages, so I don't quite remember how it felt. Then the MS Natural Keyboard Elite. I got a new one from ebay a couple of months ago, but it was manufactured in 2000 at the latest. The thing with MS boards is that the same model may have been made by different OEMs at different times (sometimes the same time). This can lead to wildly different reviews. Mine's made in Mexico, which leads me to believe the OEM was Keytronic.