Most standard (cheap flavour whatsits) mains to USB supply 500mA, as that is the standard max for a single usb port @ 5V. If you are using extra devices then you'll be needing to supply current for them as well (back of my work keyboard says 5V @ 100mA, same for my mouse)
Most external HDD's have their own power, though I wouldn't much like to run one off the RPi if it didnt, too much current draw!
Think that the best (safest and most reliable) is a powered USB hub, which you can pick up for not that much more than a non powered one (just check the supply rating, then decide if you believe it)
Could try connecting it to a coffee heater / hub, tho your coffee may be kept warmer by leaving it on top of the RPi PCB!!
I believe that the Micro USB on the Pi is actually for power only with the data pins being either not connected or ommited all together. The other USB ports are not equipped to handle input power.
The input USB (Micro B) to the Raspberry PI PCB has a resetable fuse which trips at 1.5A nominal - dead safe at 700mA. These are really to catch dead shorts
Each USB is similarly fused at 280mA trip 140mA nominal. (Raychem MINISMDC014F-2 if you want to look at the data). We looked at the more sophisticated USB port controllers and decided that they just cost to much for the project to handle.
So if you have a beefy enough supply you can suck 160-200mA out of each USB without any trouble 250mA at a push, the full 500mA (*2) would over stress the PSU that most people will use so we capped it back.
If you need more, a powered hub is the best bet. You can route that back the Micro USB B to power the RPi. As Draxy518 points out there are no data pins connected so it is OK. To get the data connection you have to wire one of the Standard USBs to the hub anyway.
On the Model A you only have one USB so a hub is needed to to get mouse and keyboard.
