In mid-2000 the talented underground rapper Skillz (you probably know him best by his year end Wrap Up songs) released "Ghostwriter", a controversial song where he revealed he was a ghostwriter for several mainstream acts.
The original version of the song censored the names. However a rare recording of a live performance shows Skillz dropping actual names (a cardinal sin for ghostwriters). According to him they never paid and that's why he called them out.
Check out the original.
Not many people know Will Smith's 90s standard, "Getting Jiggy With It", was written by Nas. At the time both artists were on Columbia records and were encouraged by their label to work together. Their first collaboration was a song Nas did for the Men in Black Soundtrack called "Escobar 97'". Later that year Will Smith approached Nas about writing a hit for his upcoming album "Big Willie Style." Nas agreed (for an undisclosed sum), sat down, wrote the song and the rest is history.