The electric bike builder marketplace is full of cottage industries. This is a list of vendors for people who are considering building an electric bike themselves. The shops below are owner operated with at most a few employees. Every vendor on this list is known to be reputable and knowledgeable. This list is current as of Summer 2012. We will add and take away names off this list as the industry develops. As of this writing these are the big dogs. If you want to read more about recommended vendors see this thread on Endless Sphere where all these vendors are mentioned. Electricbike.com has no affiliation with anyone on this list and does not accept advertisement dollars. This list is for the good of good people looking for good people to buy from some really good goodies.
Everything Dealers
These guys sell all components to build electric bikes and kits. These are one-stop stores run by highly knowledgeable owners. Ebikes.ca (Canada) The inventors of the Cycle Analyst, ebikes.ca is known to be highly knowledgeable and known for their amazing support. Nothing they sale is junk and every product they offer has been thoroughly tested. Their prices are a little higher than others on this list, especially for those not living in Canada, but way worth the extra cost. Check out the founder of ebike.ca, Justine Lemire, talk about his ideas of running a proper e-bike business in this Troy Rank Podcast: 
Standard Podcast [ 1:01:13 ] ItsElectric (Canada) Another big Canadian e-bike part supplier, ItsElectric has also been a pioneer in the development of hub motor technologies. Emissions Free This is a Chinese based company with a British owner (Cell Man) which is well known in the DIY community for providing affordable and fast motors, batteries, controllers, etc with direct from china pricing (but expensive shipping). This is an especially good place to buy a Mac (BMC clone) geared motor and controller. Cell Man is also known for providing state of the art lithium batteries. Electro ride Hosted on electric-bikes.com, Electro Ride specializes in BMC geared motors. FalconEV Known for their Crystalyte hub motors (renamed) and their well designed triangle battery bag.
Hub Motor Kits
Hub motor kits basically contain everything you need to convert a regular bicycle into an ebike. They come with hub motor, controller, throttle, torque arm, and sometimes batttery. Amped Bikes- Specializing in 9C ebike kits, and 250 watt motor kits EV Deals - As of this writing these guys have the best prices on Crystalite kits, and 9C kits. Hyena Electric Bikes Specializiing in high performance builds and 1200 watt kits.
Lithium Batteries
Ping Batteries - specializiing in LiFePO4 batteries with BMS and charger straight from China at a decent price. Hobby King A popular seller of LiPo packs without BMS protection, based in China. Only recommended for advanced knowlegable users. Ep buddy Wiring harnesses and connectors that can be used to assemble battery packs.
Mid Drive Builders Stuff
Astro Flight Make some nice but expensive electric motors that can be used for electric bikes (see our story on Astro motors on ebikes) Hobby King Providers of affordable RC motors and controllers. Fun for the E-bike guys to try to adapt this slick looking (and small) technology to ebikes. Sick Bike Parts – known especially for their freewheel crank arms, and other mandatory freewheel accessories. Mid Drive kits - See our list of 10 commercially available mid drive kits