A couple real life names were put to use on the show:
The real Lloyd Braun is Larry David's lawyer and manager.
The real Alec Berg was a writer for Seinfeld (and now for Curb Your Enthusiasm). In "The Face Painter", Berg's name is given to an attorney friend of Jerry's who gives him New York Rangers playoff tickets.
Though never seen (but frequently mentioned), Kramer's friend Bob Sacamano was modeled after writer Larry Charles' friend of the same name.
And most famously, the real J. Peterman was the founder of the actual J. Peterman Catalog, just like the one portrayed on the show.