I need a "free for commercial use" font that looks like the font used in Windows Metro.
It will be used for display on website, both titles and text. With bold and 'slim' version.
Do you know of something like that?
thomasrutter's answer is excellent; thought I'd chip in a little extra.
Segoe, Myriad, and Frutiger are in a genre of sans-serif fonts called humanist sans-serifs. Just as all rhombii are not rectangles, not all humanist fonts will look like Segoe and friends. Most won't. But this might help you in your search.
One free humanist font that is in the same ballpark as Segoe is Open Sans, which is under the Apache license. Not sure of the full implications of that license in terms of attribution, but you can use it for commercial work (Anything that's hosted on Google Web Fonts has a license that allows for commercial work according to this page).
Windows Metro uses the font Segoe.
The font Segoe bears resemblance to the font Frutiger, so in your search for Segoe alternatives you can expand your search to Frutiger alternatives.
The font M+ 2p is a free font that's an alternative to Frutiger:
It is indeed a strange name for a font, but it is desgined by a foundry called M+.
You might also want to do a search on other Frutiger, Segoe, or Myriad (another font similar to Frutiger) free alternatives.