The STMPE2401 is a multi-function device from ST Microelectronics intended for mobile, low-power applications. It provides an I²C interface for up to 24 extra GPIO lines, each with independent interrupt generation capabilities. Up to 20 of those lines can be used with the integrated 8x12 matrix keypad controller. Another 3 pins can be used for programmable PWM channels, and 3 other pins can be used for a rotator input. All of the subsystems can operate together, at the same time, and each subsystem also provides additional interrupt capabilities.
This device appears on the Peek handheld, among many others. I wrote this driver for another device which I am still unable to mention (sorry about that).
There's actually a newer chip, called the STMPE2403, I believe, which makes a couple of improvements over the original. Namely, it has an automatic sleep function that ensures the lowest power consumption possible, but it's still capable of running in 2401 compatibility mode.
In the linux kernel, this should eventually show up under drivers/mfd, with sub-components showing up in drivers/gpio/chip, drivers/input, and so on. Currently, I've only implemented the keypad functionality but have provided lots of structure for the other subsystems. Here is the patch.