I don't want to embed my project in Salesforce technologies, so basically
- no Visualforce pages
- no Force.com Sites
I do want to write my own front-end on a separate server that just makes AJAX calls to the Salesforce back-end.
In addition, I want the application to be accessible for any user, even if he/she does not have a Salesforce account. So the AJAX calls should not require that the user logs in on Salesforce. I want anonymous users to be able to retrieve public data from my organization and create new entries when it is useful (in the case of a survey for instance).
Even though these requirements generate some security concerns, I can imagine that Salesforce takes care about the requests rate limits on their API endpoints and that it is possible to restrict the access to the API on a host name base (e.g., only requests with origin host
my-trusted-domain.com should be allowed, send a 403-Forbidden otherwise). I would be surprised if SF does not provide such basic features.