Right-click on the folder you want to share and select Properties. Click the Sharing tab, and check “Share this folder on the network”.
Note: If you’re on Windows, I strongly recommend turning on some kind of anti-virus software. I’ve never gotten a virus from the network, but only because my virus scan catches ’em.
Mac OS X:
Go to System Preferences- Sharing – Services – Windows Sharing.
This may serve out your entire home directory by default which may be more than you want. If you want more fine tuned control (specific folders or what not), there’s a program called SharePoints. If SharePoints doesn’t work or offends you in some fashion, you can edit a file on your computer called: /etc/smb.conf – it’s pretty well-documented and straight forward to do so.
Get the samba tools (differs by distro, but every decent one should package it) and edit the samba conf file (typically /etc/smb.conf or /etc/samba/smb.conf) There’s probably GUI tools as well, though I know nothing about them.
Finally, here’s a list of computers that should be available:
Download, don’t stream!