Filament guide to help feed the filament off the spool at a consistent rate. Attach a PTFE tube (4mm outer diameter, 3mm inner diameter) to one side, and run it to the extruder (make sure it's long enough for the full range of the extruder). Running the filament through the tube should keep the spool from being yanked around as the extruder moves. The spool should turn at the rate the extruder feeds. I spiral the filament tube around the extruder cable a couple of times and have it attached with a velcro strap.
Update: The original part has more resistance than it should, especially when you get to the end of the spool and the diameter decreases. The new v2 version is designed to attach further away (usually the top rear bar).