This contribution adds a second, OPTIONAL Address line for customers to use. (They can use it to put apartment number, company name, whatever.) As far as I know, it is comprehensive, and the second line appears at Billing, Shipping, in the customer's account, in Admin--everywhere.
DIFFICULTY: MODERATE. It is
a pretty long modification to make, but not *difficult*. Just a bit tedious.
There are no files to copy. Run the SQL query to insert the new fields as instructed, then just find and replace where the instructions say to.
Comments are welcome: firstname.lastname@example.org