Google (Gmail) click below for more information
http://support.google.com/accounts/bin/answer.py?hl=en&answer=180744
Paypal click below for more information
https://www.paypal.com/us/cgi-bin?cmd=xpt/Marketing_CommandDriven/securitycenter/PayPalSecurityKey-outside&bn_r=o
Yahoo click below for more information
https://edit.yahoo.com/commchannel/sec_chal_manage