Occasionally questions are posted to draw them out. Web pages - Individuals with WOW personal web pages on technical topics are added. People they visit our web site technical areas are added to the list. Hire interns or temps to surf the web for names you might have missed. The same technique works at conferences and trade shows.
People are suckers for give aways.
Filling out an information sheet for a contest is a variation of the name capturing concept. Have employees ask for cards on the bus to the pickup spot, etc. In extreme cases private detectives and consumer databases can be used to capture names.