USB is doing it the hard way on both ends of the line. If you really want to go that way take a look at Jan Axelson's USB website...http://www.lvr.com/usb.htm

Both of your examples can be done simply and easily using the CP2103 breakout board from Spark Fun. You have 4 GPIO pins which you can control from the PC.

The USB Bit Whacker gives you a great deal more power. You can create your own PBP programs and download the hex file using the USB bootloader.

Both connect via a USB cable but appear like COM ports to the PC allowing simple serial commands to control them. The only drawback is they are a bit slower than pure USB because they translate between serial and USB.