Bangkok is so easy to get around that you do not really need a driver or guide. But if you want a driver then I would suggest

Ratt and her team have been highly recommended on other forums and can be reached at rat_bkk@hotmail.com or www.ratservice.com
Julie’s Taxi - see her web Home Page

If you want both a driver and guide the following are good

Tan- licensed guide izthai@gmail.com or izthai.com
Nook- licensed guide; cnnchinlawan@hotmail.com
Nok- unlicensed guide but good nan_ke80@yahoo.co.uk
Cherry. She can be contacted at magical_routes@yahoo.com

You can also arrange with the hotel to get you a driver or guide but make sure that you are satisfied with their english

I have never found a taxi driver from the airport whose english was good enough that I would hire him for a day. Nor have I heard of anybody complaining about hiring a driver/guide and having them pick up other people. Not saying it could not happen.