Is there a tool which will allow me to use all my IM identities from Yahoo | Google at the same time. I would prefer not to logout and login with a different IM identity if I have to communicate with a different set of users. Meebo doesn't allow me to do this either.
Take a look at Pidgin.
I'm assuming you're looking for a Windows/PC client.
Pidgin as mentioned - great for windows
Adium - good for OS X
Meebo - online.
I know meebo allows multiple google accounts since I use it, yahoo should also work but I don't use multiple yahoo accts personally. There were other multi-protocol clients before too that did this like Miranda, Trillian, etc. the keyword here is "multi-protocol" im client.