yahoo messenger in ubuntu

Use wine to install yahoo messenger :)

Or just use a client such as pidgin which allows you to connect to the protocol along with many others.

Yahoo messenger version for linux is available on the net http://linux.softpedia.com/get/Communications/Chat/Yahoo-Messenger-002.shtml
and further instructions are here:http://ubuntuforums.org/showthread.php?t=81895

2nd way is to install yahoo messenger using wine which is not such a good idea
http://ubuntuforums.org/showthread.php?t=205434

3rd and the best way is to use pidgin or kopete.
Pidgin: it is there by defualt in Internet>Instant messenger
configuring instructions for yahoo
http://developer.pidgin.im/wiki/Using%20Pidgin#HowdoIuseAIMMSNYahooGoogleTalkJabberXMPPICQoranyotherprotocol

Kopete : i personally prefer kopete which is the instant messenger for kde

Though I have switched completely to Ubuntu from windows, I have a huge number of friends on Yahoo messenger and regularly need features of ym such as instant messaging, chatrooms. Occasionally I also need voice and webcam.
I use a combination of Pidgin and Gyachi for these.
With PIDGIN I can use IM, access chatrooms plus I can log on to multiple yahoo accounts, google, hotmail, other accounts simultaneously. I can also display my pic or avatar. I cannot however use voice chatting, photo-sharing or webcams.
When i must use/view webcam or voice, I occasionally use GYACHI .
I hope there there is an integrated solution soon.

how can i get my cam work pidgin messnger?

@ dark_angel: That's off-topic. You should've created a new question
But see the FAQ (http://developer.pidgin.im/wiki/Using%20Pidgin#VoiceandVideoMicrophoneandWebcamSupportNotImplementedYet)