Not exactly throwaway per say, but I have a Google Voice VOIP number that I usually use when I'm signing up for services that require a phone number. Some services seem to be able to actually detect that it is a Google Voice number and will reject it, but it works the majority of the time, and I believe it has helped in keeping my main phone number from receiving as much spam. One plus about using Google Voice is that I can access it later if I need to use it to regain control of an account if I lose a password or whatever. That number predictably gets a fair amount of spam now, but another nice thing about Google Voice is that I can quickly use full text search on the entire call history, voicemail, and sent/received SMS messages.
