If you are lucky enough to have the most recent version of the Pi Model B (Rev 2), you can conduct this hack.
The Ras Pi has several headers. P1 is the GPIO pins, P2 and P3 are the JTAG pins, etc.
For this hack, we will use the two-pin P6 header.
What you need:
1x Raspberry Pi Model B Rev.2
1x 2-pole momentary pushbutton switch
For this system, simply solder the two poles of the switch to the two pins of P6.
Or, for a "remote" switch, attach two wires to P6 and then connect the switch to those wires.
To reset the RPi while it's on: hold the pushbutton down for about a second.
To wake the RPi from halt state: press the pushbutton down until the ACT light turns on.
Have fun, please comment with any issues!