Loading up the Raspberry Pi
The Raspberry Pi, out of the box, doesn't do anything. You need to hook up a monitor, a keyboard, a power supply, an Ethernet cable and you need an SD card loaded with the operating system to make it work.
The instructions at RaspberryPi.org are well written so that's a good place to start. I went to the website and downloaded the latest release of the Raspbian operating system - it's a performance tuned version of Debian Linux for the RPi. There are several OSes that you can download, but I'm most familiar with Debian and I know it installs easily and works great so I used that one.
I burned the SD card using a little USB/SD adapter and the command line interface on my Mac (solid instructions also on the RaspberryPi.org website.) I powered it up, ran through the handy configuration tool. Part of this configuration provides the ability to turn on SSH. Once I did that, I was able to unplug the keyboard, mouse and HDMI cables and connect to it using SSH, "headless", from my Mac.
WebIOPi. It's basically software that provides a web interface to the GPIO ports. Exactly what I was looking for! It's free, but there's a link where you can donate to help support the effort (donate!)
Using the instructions on the site, I loaded the software. It took maybe 10 minutes. The instructions were top notch - there's even a video how-to. When I fired up the web interface, bob's yer uncle, it worked great the first time. Using a multimeter I monitored the voltage of the GPIO pins as I flipped them on and off using the web interface. I wasted about ten minutes playing with it - hypnotic, really...
Next: wiring it up and testing