Multiple login in yahoo Without Using any SoftwareYou can login with multiple id's on the same yahoo messenger withoutany download or patch .Follow these steps :

Go to Start ----> Run . Type regedit, then enter .
Navigate to HKEY_CURRENT_USER --------> Software --->yahoo----->pager---->test
On the right page , right-click and choose new Dword value .
Rename it as Plural.
Double click and assign a decimal value of 1
Its done!!
Now close registry and restart yahoo messenger and try Multiple Login

Want to run Google Talk with multiple Gmail identities? If you have several Google Gmail accounts you also may want to run multiple instances of Google Talk This is especially important for families that share a single PC. Nothing worse than a family member signing you out so they can sign in under their own account!
Basically, to have "Google Polygamy" you need to run Google Talk with the following switch: /nomutex
Step 1: Right-click on the desktop
Step 2: Select New
Step 3: Select Shortcut
Step 4: Paste this into the text box:
"c:\program files\google\google talk\googletalk.exe" /nomutex
Step 5: Click Next and choose a shortcut name such as Google Talk1, Google Talk2, or something related to your Gmail account for easy remembering which account is which.
Step 6: Click OK a few times.