Ah, ok, if you're just wanting to do that sort of lightweight file transfer, FTP should be fine. And yes, it works over Wifi without any trouble. (The only real downside of FTP is that it is less secure than most other methods...)

And yeah, you don't need any third party applications. I just did it by going into System Preferences, selecting "Sharing" (in the "Internet & Wireless" section), and clicking on the "File Sharing" checkbox. You'll also need to hit the "Options..." button for File Sharing, in order to select the "Share files and folders using FTP" checkbox. Then, just follow the directions below that checkbox to connect to your Mac from another computer (such as the n900).