Hey,
You could use the ftp server built into Mac OS X. Create a new user account (or use your own, if you don't mind handing out your password), and place the files you want to share in that user's home directory.
Now, open system perferences and go to "Sharing", and then turn on "FTP Sharing". You will then need to forward the ftp port (TCP 21 I think) through your airport base station (using Airport Admin utility) to the computer which is hosting the server, and open the port on any firewalls you may have.
When you have done this, tell your friends your external IP address, and the user name/password for the account which contains the files. They will be able to log into your computer's ftp using Internet Explorer (by typing ftp://<your ip address> in the address bar and entering the login details when prompted). They will then be able to browse the account's home directory and drag and drop the files onto their computers.
Hope this helps,
AkleyMac