There are some lounge songs which are tingling in my mind. Enigma, Buddha Bar, Karunesh and once again Rahman's song is up there.
Enigma is just an amazing project. They really managed to make Lounge partially mainstream. Very intricate and very close to nature and most of the times, very dark. Some songs of Enigma are like "eargasms" (or if there is some word like that)....Deep Forest is another band who samples forest sounds into music.
I found the history of "Buddha Bar" (according to Wikipedia) pretty interesting :-
It is not a band, but a restaurant in Paris, famous for its ambience! They have made a franchise out of the music. But I think the music is really serious, and it needs more attention than just being played in the background.
Rehna Tu from Delhi-6. To appreciate this song and intricacies you should have a keen ear to music. Rahman sounds like Sting in this song.
Enjoy Lounge when you are in introspection mood.