G-Apps Browser provides easy access to multiple Google sites and services from a single Windows 8 app.
The app starts with a browser window, open at Google search: you can enter your search terms and browse for results just as usual.
There's also a selection of icons at the bottom of the screen, though, and clicking these will launch the appropriate site. There's Gmail, Picasa, Google Calendar, Google +, YouTube, Google Translate, Google Maps, Google Finance, Google Books, Google Drive and more.
This isn't implemented in any particularly clever way. Click one of these icons and essentially you're just getting a regular Internet Explorer window (not the Modern UI version) open at the relevant site or page. Still, it is easier to use than running separate apps for each service, and if that's something you're doing right now then it might be worth a try.
It's extremely basic, but G-Apps Browser does provide a simple way to access multiple Google services from the Windows 8 start screen