Allow ability to set static URL info

When a link field is created, we should be able to specify certain redundant information (if needed) so end users don't have to type out complete URLs. This would prevent mistakes in addresses (htp://, htpp://, ww.example.com). It would also allow a simple way to CCK things like Twitter, Facebook & Myspace pages. There should be an option to make the preceding text (http://) visible to the user or not.

Example:

Website:
[http://] [user field]

Myspace Page:
[http://www.myspace.com/] [user field]

Twitter:
[http://twitter.com/] [user field]

If this already exist would anybody mind sharing how? I've searched around to no avail. Take care.