I did look, and it seems that Bing Maps is the only alternative (with both maps and aerial views - otherwise you could mirror OpenStreetMap and do it in house.)
How many maps a month does iSpot require?
Edit: It looks like they've reduced the free credit roughly 30-fold. (I would have guestimated that iSpot was within the old free credit limits).
Edit: "Embedded Google Maps
If you do not require your Maps to be customised in any way and are happy with the way that the standard Google Maps works and looks, then you can simply find a location for your Map on Google Maps and embed it onto your page(s).
This way you will not be charged as embed requests have unlimited usage under the new Google Maps Pricing.
The only drawback from using an embedded Google Map is that it is not customisable apart from the size of the map on the page and the address the map points to." (https://www.bisongrid.uk/blog/how-to-reduce-google-maps-api-charges/)
Edit: But perhaps an embedded map is insufficient (for adding observations) as perhaps it doesn't allow you to get a location back from it. That isn't required for people who specify location by GR/lat+lon, and for people who specify locations by name an alternative location service could be used while still displaying Google maps. The old feature whereby iSpot remembered locations would have helped here.