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!