I'm very impressed with GrandCentral, a service that provides a phone proxy. You get a "virtual phone number" that you can redirect to other phones based on the caller. You can block certain numbers and have other numbers ring you directly or go to voicemail directly.
Reminds me of Butler Lampson's quote:
"All problems in computer science can be solved by another level of indirection"
Another great idea from our friends at Google.