Guitarists, violinists, horn players, and other musicians who don't have both hands available to trigger clips in Ableton Live need foot controllers, particularly those who record their instrumental playing into Ableton, then immediately play back the recording while recording another track (aka "live looping").
I have a Roland FC-300 foot controller, which I got to select patches and control parameters on my Roland VG-99 processor. I've used the FC-300 a little bit as a MIDI controller, to send on/off type messages to a synth. However, I don't see a way I can trigger Ableton clips without changing VG-99 patches, or change VG-99 patches without triggering Ableton clips if I use the same foot controller for both VG-99 and Ableton Live. So I'll probably use my FC-300 just for VG-99 control, and get a foot controller for transport control of Ableton Live, as well as triggering Live clips.
This is my humble attempt at a roundup of footpedal makers which I'll probably revisit and update from time to time:
Maker: Behringer
Product: FCB-1010
Comments: Very popular because the cost is low. The main disadvantages are the learning curve involved in setting up the FCB-1010 to work with Ableton, and the size of the unit. However, there seem to be plenty of tutorials on the web.
Maker: Gordius
Products: Little Giant Module, Little Giant Foot Controller, Little Big Giant Foot Controller
Comments: The Module adds functionality to an FCB-1010. The two foot controller products are alternatives to the FCB-1010. The advantages appear to be smaller footprints and bigger feature set than the FCB-1010. The main disadvantages are higher costs.
Maker: Liquid Foot
Products: LF+ series foot controllers
Comments: Like Gordius' products, these are quite a bit more expensive than the FCB-1010. On the other hand, the LF+ 12+ and LF+ JR controllers have colored LCD displays next to each button so you know at a glance what each button is assigned to, which alone might be good enough reason to spend the extra cash. Helps to see which buttons are for enabling recording, start/stop, and for individual Live clips. No price however for the LF+ JR, which is probably the most appropriate option for me.
Maker: Keith McMillen Instruments (KMI)
Products: SoftStep, 12 Step
Comments: Both of these controllers use rubbery foot switches instead of metal switches like the other controllers. SoftStep looked intriguing at first because each of the switches are sensitive to both direction and pressure. However, early adopters have reported these switches being harder to "play" in practice - not that easy to play guitar and keep track of in what direction you're pressing your foot on the pedal at the same time. Also, some users complained of the response of the pedals compared to metal foot switches, which is critical for live looping. 12 Step looks more practical to use - it can be used for triggering clips, and also MIDI/OSC note data with aftertouch and velocity. Both KMI controllers are light, in comparison to the FCB-1010 and cost only about $100 more.
Maker: X-Tempo Zone
Product: POK
Comments: 8-pedal wireless DAW controller. The main advantage is the POK being wireless.