Single Sign On is always a security risk. Storing your passwords anywhere but your brain is a security risk. Having the same password for multiple accounts/applications is a security risk.
Yeah I know. The sad truth is, you cannot have both, security AND comfort. These are ALWAYS mutually exclusive. So take your pick.
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Although I wouldn't say that storing a user ID and password in my particular brain V 0.5 is exactly secure (well it's actually hyper secure as it locks me out too) .... fortunately most sites have a button to click that is specifically designed for such a case as my head
Don't store your passwords. Hash them. There are many nice browser add-ons that generate unique passwords per domain based on your master password. Because they hash your master password with the domain name, no password gets stored, nothing goes via the cloud and you only need to remember ONE strong password.
Of course all you need is Keepass
Why use some browser plugin to handle your passwords when you can just do it all locally, including a password generator and a handy auto login thing ? Not safe you say ? What is safe nowadays, I asketh thou :O