If you're asking about the Web application aspect of a Google service, most likely you're going to want Web Applications. See, for instance, questions tagged 'gmail'.
There are some exceptions, however.
- Google Analytics is better on Webmasters using the [google-analytics] tag
- Search Engine Optimization also belongs on Webmasters using the [seo] tag
- Google Drive file sync goes on Super User using the [google-drive] tag
- Smartphone apps go on their respective sites: Android Enthusiasts, Ask Different (Apple), Windows Phone
