Expanded version of what mende1 said
iChat on Macs up to the Lion (OS X 10.7.x) Install can do AIM and Jabber chats.
At the OS X 10.7.x and the iChat 6 version you can also do Yahoo Text chats.
While the iPad will not be able to use Video (In these chats) there are various Apps for "Mobile" devices that will be able to help.
In addition you can login to AIM Express and text chat from a Browser.
The same can be done by logging in to Google and Facebook and chatting via their Jabber services.
Enabling "Talk" on your Google Account enables Googletalk which is a Jabber server.
It is the same for the Facebook "Chat" option that needs to be Enabled as the run a Jabber server as well.
I presume there is a Yahoo Mobile version or a web Browser login that you can use.
Chats are essentially AIM to AIM or Jabber to Jabber (Yahoo to Yahoo).
The Messages App on the iPhone/iPad/iPod Touch needs to have a user on another iOS device or using an iMessages account set up in Messages in Mountain Lion (OS X 10.8.x)
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
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