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What printers (laserjet, inkjet, all-in-one) are out there that are compatible with Mountain Lion?
Fairly broad question. Do you have specific printers in mind? If so, the manufacturer's website or a google search (which will often lead to the manufacturer's website) should give you the info you need. My HP Workforce 8600 works great.
 
What printers (laserjet, inkjet, all-in-one) are out there that are compatible with Mountain Lion?

It'd probably be easier to ask what isn't compatible. Pretty much any consumer/small office printer currently on the market from any mainstream provider will work.

I'm currently using a Brother 2170w laser printer, Epson NX625 AIO and Epson R3000 photographic inkjet with ML, no problems.
 
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