Yammy (Yahoo Messenger Archive Decoder) allows you to decode and view not only yours but, also other stored YM conversations. You don't even need a password!
[Yammy 0.7] Main Window
[Yammy 0.7] Yammy in your language
[Yammy 0.7] Yammy in your language (2)
[Yammy 0.9] Listing conversations for user
[Yammy 0.9] Main Window
[Yammy 0.9] Showing Help