One of my first forays into clean transportation was with an affordable, used 50cc scooter. I always thought scooters were pretty cool, and heard they got great fuel economy.
After a week of riding on something so loud with with such vibration AND burned blue exhaust (it was 2-stroke!)
I decided it was about the least eco-friendly thing out there, and wasn't nearly as much fun as I thought because of the noise and vibration.
I later built my electric motorcycle, which was about a $2000 project, in case you were wondering, but it's a REAL vehicle with nearly NO noise or vibration, and a BLAST to ride.
I really like the idea of an electric MOPED, which as Ryland said, basically would qualify as an electric bike. (Could take on bike trails, park on sidewalk, etc.)
Not that I need any more projects, but I'd definately consider taking a moped and converting it to pedal/electric or build from scratch a hard-core electric bike.
Also, there ARE electric mopeds commercially available. And yes, they are usually made in China, and I can't vouch for quality. I think they are fairly affordable. If you buy something like that, just check on VIN, registration, title, etc. BEFORE you buy.
Always make sure you have your paperwork lined up BEFORE starting a custom project, but I'm also starting to see where people are able to register or title smaller vehicles (like a made-in-China e-scooter) because it didn't come with a factory VIN plate or other needed legalities.