I have a couple of old pens from my Lenovo Thinkpad X61t, they work fine with the SP (the stylus, button, and eraser head all function ok). Although I haven't tested, presumably comparable Lenovo pens from later ThinkPad models would also work. I usually use the stylus that MS provided with the SP, but the Lenovo pen is a nice backup (and a bit more rubbery rather than plasticky feel to it).