Gmail Chat lets you chat with AIM users even if you don't have an AIM account.
Once you sign into your Gmail account you can add AIM contacts directly into Gmail just like you would add a Gmail or Google Talk contact. Your AIM buddies will populate your Contacts list, and are searchable just like your Google Talk contacts.
To invite them to chat simply type their screen name into your Chat roster's search field (i.e. email@example.com) and click Invite to chat. Once your contact accepts your invitation you'll be able to start chatting.