I know for a fact that JayZ does, or has, freestyled songs for albums. For example, if u really listen 2 him, he leaves one point to go to another point real quick. Thats how I do. I used to go to tha studio, freestyle about 10 takes, sit down pick out 16 of tha hottest lines, put it together, and dish out tha song. I mean, its a different form of freestyling a song, but I will freestyle it.

Some artist that do freestyle are:

Biggie
JayZ
Nas(Hott)
DMX(Tha Best 4 Past 3 years)
Jin
Poster Boy
Fabolous
Jadakiss
Beans