How to share a local folder with anyone over internet?

How much storage do you need? Dropbox is ideally suited for what you describe.

 

I use Dropbox to share work files with several colleagues. So simple and as the data is also stored locally you're not really tied to Dropbox if you don't like it.

Only potential problem is network monitoring software which some companies use to stop access to various Internet sites and I've worked with some client who block Dropbox.

 

That's what FTP servers are for.

Enabling Remote Login in System Preferences -> Sharing sets up an SFTP Server. That's the SSH File Transfer Protocol and it usually runs over TCP port 22 in case you need to set up port forwarding.
To access it:

Code:

sftp <hostname>

or

Plain FTP (Leopard and Snow Leopard)
(Lion -> sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist)