Multi yahoo messenger & google talk without any patch or any software!!!! Yahoo Messenger:
Follow these steps
1. Go to Start —-> Run . Type regedit, then enter .
2.Navigate to HKEY_CURRENT_USER ——–> Software —>yahoo —–>pager—->Test
3.On the right pane , right-click and choose new Dword value .
4.Rename it as Plural.
5.Double click and assign a decimal value of  1.

Restart It's done!! Now close registry and restart yahoo messenger .

For signing in with new id open another messenger.

Goolge Talk: Follow these steps:
1. Go to the properties of the google talk Start -> gtalk -> Right Click & select properties
2. Under "Target" change the last word with /nomutex that is in place of /startup u hav to write /nomutex thas it.

i.e Target:   "C:\Users\sujit\AppData\Roaming\Google\Google Talk\googletalk.exe" /startmenu 
It should be: Target:   "C:\Users\sujit\AppData\Roaming\Google\Google Talk\googletalk.exe" /nomutex

Your gtalk is now multi login. Enjoy!