We use foot pedals to control the master brightness levels of multiple banks of effects, so we need pedals with USB inputs. The Behringer FCB1010 at $149 provides an off-the-shelf solution, with 2 expression pedals plus 10 switches that can assign the pedals to different banks. The FCB1010 generates a USB MIDI output, and our system can accept MIDI controls.
We wanted more pedals, so we eventually switched to using 4 Yamaha FC7 pedals (@$49) which have better precision and adjustability as well as a nicer feel. These are analog pedals, so we had to use an A/D converter with USB output. We tried using cheaper A/D's, but most of these used the win serial port interface, which is not very reliable - we frequently had to restart the system to reset the Windows USB. We switched to using a Labjack U3-LV ($108) as our A/D and have not had any problems afterward. We bought a cheap ($16) regulated 3VDC supply to provide the reference voltage across the FC7's potentiometer as the USB 5V supply is noisy.