For my work, hobby and business, I need COMPUTING POWER! However, my needs are kinda picky and it took me many days to narrow down the 'ideal' hacking computer. Here is what my specifications were

1. Must have hardware/motherboard parallel port (printer port). Parallel ports are the pinkie toe of electronics hacking. Youd think they'd be gone by now but nooo, they're still there, hanging around and often necessary for using older software/hardware/schems. I use them a lot for talking to laser and label printers, CPLD/FPGA programmers, bitbanging all sorts of stuff, programming chips via PonyProg, etc.. USB-parallel converters aren't good enough due to the slowness from the USB layer. Hardware parallel ports are just damn handy!
2. Must have hardware/motherboard serial port (COM/Modem port). Two if you can. These are more common than parallel ports. You can use USB-serial ports for most things but sometimes you need the hardware speed of an onboard serial port especially if you're doing some funky bitbanging.
3. Should be small, we dont got a lot of room here at adafruit.
4. Lots of USB ports. Both on the outside and on the motherboard. Especially for the shipping computer theres just tons of stuff that needs to plug in - programmers, barcode scanners, scales, backup usb keys, Arduinos. You can also use hubs.