Missed this thread first time round.
Paul Whelan's sites under the
Biology.ie banner (particularly
Nature's Calendar) and Irish Roadkill have been doing this for several years. AFAIK he uses the GoogleMaps API. So they might be worth looking at for ideas.
I have tried to install Indicia myself, but have run into problems with various dependencies (in particular it does not run on more recent versions of PHP). It's probably far too complex and heavyweight for the basic recording of point sightings. At some stage I mean to start a thread about this software suite.
For a map interface I would choose OpenLayers, either raw or in its OpenSpace manifestation used by the Ordnance Survey. You can display a range of maps (Google, OS OpenSpace, OS OpenData, OpenStreetMap, Bing) appropriate to your purpose. Indicia uses OpenLayers and a PostGIS database.
For a fairly simple display points & add enabling users to add further points
OpenStreetBugs might be a good code base to start from. Source is available on Github here:
https://github.com/emka/openstreetbugs.