How to get more than 100mA from a USB port
I think I can attempt to clear this up.
USB-100mA
USB by default will deliver 100mA of current (it is 500mW power because we know it is 5v, right?) to a device. This is the most you can pull from a USB hub that does not have its own power supply, as they never offer more than 4 ports and keep a greedy 100mA for themselves.
Some computers that are cheaply built will use an bus-powered hub(all of your USB connections share the same 500mA source and the electronics acting as a hub use that source also) internally to increase the number of USB ports and to save a small amount of money. This can be frustrating, but you can always be guaranteed 100mA.
USB-500mA
When a device is connected it goes through enumeration. This is not a trivial process and can be seen in detail on Jan Axelson's site. As you can see this is a long process, but a chip from a company like FTDI will handle the hard part for you. They discuss enumeration in one of their app notes.
Near the end of enumeration you setup device parameters. Very specifically the configuration descriptors. If you look on this website they will show you all of the different pieces that can be set. It shows that you can get right up to 500mA of power requested. This is what you can expect from a computer. You can get FTDI chips to handle this for you, which is nice, as you only have to treat the chip as a serial line.
USB-1.8A
This is where things get interesting. You can purchase a charger that does outlet to USB at the store. This is a USB charging port. your computer does not supply these, and your device must be able to recognize it.
First, to get the best information about USB, you sometimes have to bite the bullet and go to the people whom write the spec. I found great information about the USB charging spec here. The link on the page that is useful is the link for battery charging. This link seems to be tied to revision number, so I have linked both in case the revision is updated people can still access the information.
Now, what does this mean. if you open up the batt_charging PDF and jump to chapter three they go into charging ports. Specifically 3.2.1 explains how this is gone about. Now they keep it very technical, but the key point is simple. A USB charging port places a termination resistance between D+ and D-. I would like to copy out the chapter that explains it, but it is a secured PDF and I cannot copy it out without retyping it.
Summing it up
You may pull 100mA from a computer port. You may pull 500mA after enumeration and setting the correct configuration. Computers vary their enforcement, as many others have said, but most I have had experience with will try to stop you. If you violate this, you may also damage a poorly design computer (Davr is spot on there, this is poor practice). You may pull up to 1.8A from a charging port, but this is a rare case where the port tells you something. You have to check for this and when it is verified you may do it. This is the same as buying a wall adapter, but you get to use a USB cable and USB port.
Why use the charging spec? So that when my phone dies, my charger charges it quickly, but if I do not have my charger I may pull power from a computer, while using the same hardware port to communicate files and information with my computer.
Please let me know if there is anything I can add.
You can negotiate for 500mA with one of the FTDI USB chips, here's a forum post that talks about it. If you use some other chip, you still need something that can speak the USB protocol and tell the PC that it wants 500mA. Please don't violate the USB spec, your device might work fine for some people, but then it wont work for others, and you risk causing damage to poorly designed PC's.
The theoretical current limit on a bus-powered USB port (the kind in your computer) is 100mA, negotiable up to 500mA. In practice, you probably won't get as much from the ports on your computer. If you use a self-powered USB hub however, you should be able to get 500mA from every port on the HUB. So if your HUB has 4 ports, you can get 4 * 500mA = 2A.
Beware: make sure your HUB comes with a decent power adapter. Some hubs come with a 1A adapter, because manufacturers think it's an unrealistic scenario for users to draw 500mA from EVERY port.
Check out this wiki article: http://en.wikipedia.org/wiki/USB_hub#Power