Just starting using Montserrat, which is a free font that google supports. It's very similar to proxima-nova, which costs money and is used in a number of up and coming sites.
In no particular order:
- Roboto
- Roboto Condensed
- Source Sans Pro
- League Gothic
Slightly off topic
Helvetica Neue Light - iOS Development
Source Sans Pro
Times new roman
Verdana
Avería
LATO!
Even if for some reason I had problems installing it on my Ubuntu machine, I'm starting to love Lato too.