I agree with most of the previous posts - some of the suggestions are outright classics. Here are a couple of laid back and mellow recordings for late night that no one has mentioned yet:
Ahmad Jamal - Cross country Tour (recorded live in night clubs)
Modern Jazz Quartet - Live at the Lighthouse
Miles Davis - In A Silent Way (Studio)
Lee Konitz - Inside Hi-Fi (Studio)
Norris Turney - Big, Sweet & Blue (Studio)
Getz/Gilberto - featuring Antonio Carlos Jobim (Studio)
Ahmad Jamal - Cross country Tour (recorded live in night clubs)
Modern Jazz Quartet - Live at the Lighthouse
Miles Davis - In A Silent Way (Studio)
Lee Konitz - Inside Hi-Fi (Studio)
Norris Turney - Big, Sweet & Blue (Studio)
Getz/Gilberto - featuring Antonio Carlos Jobim (Studio)