A Jabber client for a smartphone. The initial client would simply be an IM client, but after the first iteration or so I would like to see the Google Talk IP Telephony stuff be used with your mobile.
An OWA notifier for a smartphone (see a apttern here?).
A suite of applications for different platforms that will “tell” people where you are without having to call, email, look, etc. There could be a client on your desktop, laptop, mobile, etc and the client owuld update a master server with your location and/or status via a web service. I’m not sure how to handle the location aspect from a mobile client and still be free though. I think mapping cell towers to GPS coordinates would not be a free service. I don’t actually think I’d be able to get the cell tower anyway, so it probably doesn’t matter. It would probably be a category something more like “in GPRS range” or something.
Most of this interest has been sparked by my purchase of a Audiovox SMT5600, which I love.