Aileen is correct. Trulia only displays residential properties in our search results at this time.
In regards to your question about what is fee-based and what is free, I can tell you that it is completely free to add listings to the Trulia search engine. Fees only come into play if you want to more prominently feature your listing within Trulia's search results or brand your listings with your company logo.
For more specifics about advertising options please access the link below.
There are fees for banner ads, but not submission of listing data. It is my understanding that Trulia intends to charge for featured lisitngs or premium positioning at some time in the future. Please take that as a question to Trulia, rather than a statement. I have defintitely heard that and wish to confirm.