I want to modify Links 2.0 in a way that every time one user adds a link to my database he can supply his website URL instead of his email address.

I've tried to modify the link.html file replacing mailto: with http:// but every time i wanna modify one link from the admin pannel, if I enter one url in the "user email" location, I get the error message:
Invalid Email Format.

Thanks for your help.

Even if you do not anticipate using the email address now, I would still comment out the email field in links.def and forms add.html and modify.html so that you can add it back in later. As for adding a new field for a website, see this FAQ...
http://www.gossamer-threads.com/...es/Detailed/162.html