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You are opening yourself up for a huge number of risks, by passing a username and password to remote sites you fetch. Not only does that pass the information across your network, your ISP's network, their backbone provider's network, and so on in the clear, but others can capture and misuse it for their own maliscious deeds.
Even if you don't care if someone has the username and password for your website logins, you've now given maliscious users one more item of information about you, one of your usernames, that they can try to exploit elsewhere, such as other websites that you might visit frequently.
I'd strongly advise against what you're asking to do.
Once you are willing to accept that risk (and I wouldn't be, I use a different password for every account I'm responsible for, changed every 30 days, religiously), you have to realize that this will only work with a limited number of sites.
Some sites store the login information in browser cookies, others in session cookies, others behind SSL, others pass the info in the URI field of the browser, and yet others, it is passed in form fields via a POST to the remote server.
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