The Neptunes created the beats for Timberlake's most popular solo songs, two of which are called "Like I Love You" and "Rock Your Body" (not sure if those are exactly the right names). I wish the Neptunes would have taken the beats from those two songs that they sold to Timberlake and used them for their own songs (and inserted their own vocals, as Timberlake's voice is annoying as hell IMO).
There are two guys on the Neptunes (Pharrell and Chap) that do all the beats and, like others here have said, are freaking geniuses. Timberlake's solo career wouldn't be half as successful as it has been had it not been for the Neptunes. AFAIK, the Neptunes have produced beats for people/groups like No Doubt, ODB, Jay-Z, Usher, and Blige (among others I can't remember).
Btw, John Mayer is great.