I haven't done one of these in a while, but I think I came up with a good one on the train.
Why isn't anyone doing free VOIP-based, ad supported wakeup callsand meeting reminders?
It seems so easy... Create a little plugin in Outlook or even easier, cc an e-mail address for the wakeup service and let it call you using a VOIP backend, but with a relevent advertisement as part of the message. You could generate very targeted ads depending on the time, keywords in the event, location, or even letting the carrier tip you off on your geolocation on the call.
"Its 6AM... time to wake up for your 9AM Meeting with Jim. For a quick pick me up, drop by the Jamba Juice on 22nd and 5th for a Bright Eyed and Blueberry. Tell them we sent you and receive 10% off!"
Just the wakeup calls alone could generate lots of ads from Jamba, Starbucks, Dunkin Donuts, 7Eleven, etc. Throw in 1-800 Flowers and FTD for Birthdays, Anniversaries, etc.
Plus, the neat thing is that you could private label it for anyone with a quick paste of some HTML. Conferences could use it, too.
Knowing where someone is or needs to be, the context of what they'll be doing, and being able to reach them at an appropriate time with an opt-in call should make for great ad potential. Its probably not too hard to setup either.
Feel free to take this plan and build a company. Its all yours.