Yahoo Messenger can be used withoutht downloading huge file & installing it.You can try Yahoo Web Messenger @ http://messenger.yahoo.com/webmsgr/fmsgr.php. But sometimes this does’t work properly.Its completely not working in my system.
Another better way is www.e-messenger.net which is very good.In this site you can also use MSN Messenger.These messengers are not fully functional as normal one.But its good for chatting purpose.This will be helpfull to working people where they don’t have privilages or permissions to install any programs