After struggling with the limited features of the built in Snow Leopard server FTP service, I started looking for 3rd party FTP server software. Specifically I was looking for the ability to easily lock particular users into seeing only certain folders and also the ability to use secure protocols such as SFTP and FTPS.

Compared to the other options out there, CrushFTP is far and above the best software available for the mac -- especially for the price.

Some of the user modification interface is confusing, but once you get the hang of it it works well. As a major bonus, the developer makes himself available to fix bugs and troubleshoot issues should they arise.