This Greasemonkey user script shows a button at the top middle of every Google Service, and allows you to easily change between your favorites Google services in the same window (not like this pseudo-launcher at top-left of Gmail and GCalendar).
The services do you want to use are customizable, just editing a little part of the code.

You need Greasemonkey and... you know how it is...

Click right button in this link and go to Install User Script: GoogleLaunch.user.js
And you're ready to go to any Google page and will show something like this: 

You will see this button in each Google page:

After click the button:

Enjoy.

Suggestions?

lardissone  AT  gmail  DOT  com

Thanks to the fantastic jQuery library!