iPhone certainly was the most visible, in that I use it far more in any given day than any other purchase I made this year. In terms of practicality per hour, the twin AirPort Extreme base stations have to win. Having a good home network, wireless printing (with multiple printers thanks a USB hub), and wireless disks has saved me unknown amounts of time and effort. Rounding out third would be Leopard, but since its only a recent purchase I can't say for certain if it was a great investment. File sharing between Macs has been vastly improved, but the AirPort Disk feature carries most of the credit on that one.