On my site, personal web sites are ok but no business websites unless they sign up to be an advertiser. If someone is mentioning a business, that is ok but if they work there or are affiliated with them, it is not allowed.

You're right, allowing URL can be hard because there are those who provide them only to get users, but it can also be a resource for someone. We can't provide everything as much as we would like to. Some suggestions can be:
1. Provide an area for it, or
2. Give specific rules on how it can be used.

On my site we have a classified section to allow businesses and personal sites (Items they make) to post their URL, otherwise in our forums the signature link is disabled, except for a section for forum owners.

Also people can always opt to PM if they are truly trying to be helpful.

Good point!