SlyDial: Leave voicemail without actually having to talk to anyone
Slydial lets you make a call to any landline or cellphone number in the US and go straight to voicemail. Just call 267-slydial (that's a Pennsylvania area code, in case anyone was wondering), and while listening to the somewhat amusing, somewhat offensive introduction, just enter the number of the person you want to call. Slydial will play a rather obnoxious ad and then connect you to the voicemail. The person you're calling will then get a missed call and/or voicemail message, but their phone will never ring. It's sort of like sending a text message. But with voice. And without the SMS fees.
If you want to avoid the obnoxious advertisement part of the equation, you can also register for an account and pay $.15 per call or $4.95 a month for unlimited calls. You know, in case you like to leave a *lot* of voicemails, but can't stand actually speaking with people in real-time.