The software company's know that people know that people search for "free", that's why they use "free to download".
I'd suggest trying terms like "freeware" or "open source" to see if that helps at all with your searches.
But, in any case, the people that are selling software are pretty crafty as to how they set up their web sites for bot searches, so you are likely to get unwanted results even using the above suggestion.
You may also want to use http://download.com
(cnet dowloads) and http://sourceforge.net/
when looking for truly free software.