Maybe more of a naive question than an enterpreneurial (or even hackerish) one, but here we go.
In 2005, out of pure boredom (and as an exercise to learn a new programming language), I came up with a Google Maps mashup. The idea here would be to allow visitors to click on the map and leave a funny little icon, a message, a link, whatever. Each site owner can setup their own map. The reception was very good (since I only expected to use the service myself): around 10k people signed up on the first month, and a lot more followed (it eventually declined over time, probably because I just left it there collecting dust!), but the fact is the site is still active today, still brings a lot of 'thank you so much for your little service' emails to my inbox every week... And gives me a feeling that I'm leaving money on the table here.
Funny incidental story: one of the 'competitors', Frappr, was actually one of the first 100 MyGuestmap users. Their story, though, is a completely different one: http://mashable.com/2007/10/18/platial-frappr/ (execution is way better too, I admit!)
With regards to business model, it's basically 50/50 between adsense and donations (I get some incredible donations from time to time, and a relatively large number of small ones).
Question is: what can I do to get the service out of its current letargy and maybe make it a viable side-business (not planning on making a living out of it, really)?
oh, the site is at http://www.mapservices.org/myguestmap (I also bought myguestmap.org a couple of years ago, but just redirect to mapservices.org for pagerank reasons) - any other comments (positive or negative) are also welcome!
That is a virtual certainty, considering you are not charging money.
My advice is to consider splitting the service into two levels (does the Maps API let you make money with it? I guess that is moot since you're already doing it) and selling some portion of the people on the upgrade. It is a rare, rare, rare application that will take in more in donations than it will in payments. One useful feature might be allowing embedding in other people's websites. (Remember, with a freemium application you need to make the -mium something that solves a pain point for people. "I want to offer maps" doesn't strike me as an obvious pain point, but given that people are actually using this, "I want people to not leave my site" does strike me as worth paying for for a business.)
Don't be put off by free competition -- I have scads. They're wonderful people, and they display AdSense ads for people charging for similar things. i.e. you.