A few people in the Useless thread including myself had issues using the PS3 controller on the PC but MotioninJoy makes the process fairly painless although cryptc guides / youtube videos don't show it. I hereby declare it Useful so here’s how to get your PS3 controller running on your PC for comfy couch gaming. Skip step 5 if you don't want to use bluetooth.
1) Download
MotioninJoy, I got v6.3, admire its garish screen:

Optional: There's a local mode so you don't have to be connected to the internet by downloading the file
here and extracting it to the ds3 folder in the MotioninJoy installation folder. You should use this mode. Just in case you ignore this advice I'll put the non-local mode menu names in [aka brackets].
2) Plug in your bluetooth
dongle adapter if you haven’t previously done so and let Windows load stuff. Plug in your PS3 controller via usb and let Windows load more stuff.
3) In MotioninJoy, click on Install [aka Driver Manager] and you should see the controller and bluetooth hardware id’s. If those are the only two items plugged into your computer then click install all, otherwise check the appropriate boxes and click load driver. Success messages should follow.
4) Click on Start [aka Profiles] and select a mode (I use Xbox 360 Controller Emulator for Witcher 2), click enable, then click vibration testing. Rumble! You can click on Game Controller Panel and test out the buttons / sticks.
5) Bluetooth Step: Click on Bluetooth [aka BluetoothPair] and your adapter should be listed. If not, unplug and replug it / use device manager. If you see the Manufacturer as Cambridge Silicon Radio then it should work with the PS3 controller. If not, click on supports feature and if the Enhanced Data Rate ACL options are checked then you should be fine. For reference my adapter is a cheapy Bluetooth v2.1 + EDR. Click on Pair Now and unplug the PS3 controller. The controller led should be lit indicating battery charge level. Go back to Profiles and click enable / vibration testing for verification (battery level is also displayed here).
That’s it! To turn off the PS3 controller with bluetooth (it will automatically shut off after inactivity), either hold the controller's home button down until it turns off or go to Profiles and click disconnect. To reconnect via bluetooth, just press the controller's home button. When you reboot you can reconnect the PS3 controller just by pressing its home button but if you want the analog sticks to work then you have to open MotioninJoy and click enable with an analog stick mode selected (as I mentioned I use Xbox 360 Controller Emulator). After doing that as long as you computer is on you can reconnect your controller multiple times and the analog sticks remain enabled.
There's no need to keep the program running after enabling the emulator mode after a reboot, and I turned off MotioninJoy starting with Windows since it acted weird recognizing bluetooth after boot. If you ever want to use bluetooth with another device (phone, keyboard, etc.) just go to Install [aka Driver Manager] and select the bluetooth box and click Roll Back. Then reinstall the driver to get the PS3 controller working again.