If you both have iChat or Messages you can use it's Screen Sharing abilities.
You can do this AIM to AIM (Screen Names that are valid with the AIM service which includes some Apple Issued Apple ID names - although this can be version dependent)
You can also do it Jabber ID to Jabber ID.
Jabber IDs include Google Mail IDs set up for Google talk and Facebook Accounts with Chat Enabled.
Check you can Video chat with the person as it takes a similar amount of Bandwidth (the bit of the Internet Speed you computer gets).
You needs to Enable Screen Sharing in the Video Menu of iChat or the Messages app in Mountain Lion.
Highlight the Buddy in the Buddy list and use the icon at the bottom of the Buddy list (It will offer your to them or them to you)
You can also use the Buddies menu when a Buddy is highlighted.
It is also possible to "switch" to Screen Sharing in a Video chat.
In reality it instantly closes the Video chat and Starts the Screen Share.
It does not have the ability to "Switch" back as closing the Screen Share close the connection.
Screen Sharing includes a Audio Chat alongside (Technically it is an Audio Chat with an Apple Remote Desktop type VNC connection. The Apple Remote Desktop engine provides the "Desktop" "Pics").
To do this VNC part you router will need to be using UPnP (Or Port Mapping Protocol in Apple Base Stations)
AS Video chats and Audio chats use the same ports to connect it means that if you can Video your can Screen Share
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously