Say you got a table with 1000 records in then realize you did something real stupid(like I did) and not create a key or keyable compound fields. Welp you have to add an identity key but there's no easy way to back fill the entire 1000 records... Until now

step 1.) Script the table to create a new one with a slightly different name.

step 2.) add an identity key, seed it to 1

step 3.) insert all records from the old table into the new one. SQL server will fill the identity column just like you need them

step 4.) drop the old table without a key

step 5.) rename the new table

step 6.) go have a beer! you're done