using cookies an safety
Well you can always write a cookie which remembers the username and password, but by doing so there are some safety issues you should consider....
You simply can not expect that a user always visit your site from behind his own computer, neither can you expect that a user is the only user on that particular machine.
Leaving cookies with the password, even encrypted is generally a bad idea. Also, permitting the browser to remember the password is also dangerous and it could seriously compremise the safety of your site.
Why? Well take a look around on the internet, lots of software to crack (even encrypted) cookies etc. etc. So if you have a cookie tool beware that nothing is safe. Leaving the option open that a browser can remember passwords could be easy for the user, but you can't expect that every user of your site is aware of safety issues.... As a web designer you are responsible for that safety issues, protecting peoples privacy and money in case you have a webshop etc.