Is there an open-source alternative for the FF Netto font?

You could do with more explanation as to what you're really looking for. Typewriter-esque? Roundedness? No stroke contrast? High x-height? Lining figures, i.e. all numbers are same height? There are many aspects to a font and we have no clue which ones you want.

Also, you ask for open source, but I have a hunch what you're looking for is actually free (as in money and licencing) instead of with access to sourcecode. In which case; have a look at this selection over at fontsquirrel.com. That site only lists fonts which are okay to use for free, in commercial projects.