Lifehacker has an excellent roundup of all the things Google Voice can do other than, you know, make phone calls. My personal favorite is the call screening bit:
Its not really a hidden or even obscure feature of Google Voice, but the depth of call screening, filtering, and blocking in the service likely goes unexplored. If youre not dedicated to your Voice number, you can have your contacts keep calling your trusted number. Your Voice number? Give it out to companies or pepole you know will be phone-spamming you, or use it on personal business card or discrete slips of paper. You can have Voice make each caller say their name and present it for you before taking the call, and set offenders of your free time to always go to a specific voicemail. Truly egregious jerks can even be sent to what sounds like a wrong number. Now, that—that is sweet justice.
I love this feature of Google Voice. I give out the number instead of my cell number, and then can either screen those calls straight to my voicemail, or only have it ring my phone at a particular time of day. That way I’m never caught at a bar when someone calls who I shouldn’t be talking to while in a bar.