Source Sans Pro is Adobe's first open source typeface and was designed by Paul D Hunt. It was specifically designed for user interfaces, and is available in multiple weights. I think the regular weight also works well for body text.

https://store1.adobe.com/cfusion/store/html/index.cfm?event=...

The font is also available on Google fonts

Shameless plug: I use Source Sans Pro on my website if you want to see an example of what it looks like for body text and headlines :-)

http://www.interfacesketch.com/