What simple IC can I use to extract 500mA from a computer USB port?
Consider Atmel's ATtiny85 with V-USB. It's an 8 pin AVR chip that you would have to program with V-USB, which is a software-level USB implementation that would enable the Enumeration Phase, which would allow you to use the entire 500mA available.

It's about a $1.50 in SOIC packaging, pictured below, which saves both space and cost:



Easily programmable and inexpensive, whereas the FTDI chip above (FT232R) is about $6 for one. 

If you want to communicate with the chip, using V-USB also gives you the ability to act as a CDC-class USB device, which is akin to a serial port (UART), just like the FTDI chip.