One Tree Hill - The concept of music is a pretty large component of OTH - Peyton's record label, Haley's music career and the incorporation of actual singers and groups into the show's plot (from Fallout Boy to "Mia") - so it's no wonder that the series has a knack for picking out some fantastic music. One Tree Hill has some excellent music supervisors - choosing music that is not so mainstream that you'll groan at hearing the same song on yet another series, but mainstream enough that the show's young audience will embrace it. OTH has also used real life bands in a creative way - once incorporating a "Benefit Concert" into an episode, only to sell an actual benefit CD, with some of the proceeds going to charity.
Some bands used in the series that I'm fond of: Nada Surf, Jack's Mannequin, The Weepies, Citizen Cope and of course, Gavin DeGraw (back when they still played the theme song).
Supernatural - Seriously, would some of the things that the Winchester brothers do seem nearly as badass were it not for the sounds of AC/DC, Kansas, Creedence Clearwater Revival, Bon Jovi, REO Speedwagon et al that fill the background? If I ever find myself fighting against forces from Hell, I can only hope that my fights will be set to the same awesome backdrop of classic rock. I would argue that no other show is as faithful to such a specific genre of music as Supernatural, but it's a perfect match. I don't want to know what the music licensing fees are for this one...
Veronica Mars - From the moment that I first heard usage of the Dandy Warhols' "We Used to Be Friends" as the Veronica Mars theme song (which perfectly described Veronica's outcast status in Neptune), I knew that I would approve of the show's music. Much like the character of Veronica and the series as a whole, musical selections tended to be slightly offbeat and slightly alternative. Awesome bands used in the series: Spoon, Tegan and Sara, Stereophonics, Interpol, Old 97's, The Hives, Starsailor and Sparklehorse.
Scrubs - Scrubs probably has the best soundtrack that I've ever heard on a TV comedy. In fact, I'd say that it's the apt musical selections that often contribute to defining both the show's most comedic moments and its most serious ones. The Fray's "How to Save a Life" played at the end of the episode where Doctor Cox lost 3 patients in a single day and decided to quit medicine. It was one of the most powerful episodes of the series and I credit usage of the song for helping to set the tone. Of course, lets also not forget some of the show's lighter musical moments, including Turk's Air Guitaring to Boston and the a capella sounds of Ted's Band.
Grey's Anatomy - You might argue that no show so much as Grey's has had the ability to catapult a previously lesser-known group into musical superstardom. "The Fray" and "Snow Patrol" were once upon a time not household band names, and it was before their songs appeared in key scenes of key episodes of Grey's. In fact, "Chasing Cars" from Snow Patrol has become so closely associated with Izzie bawling over a Dead Denny that I can no longer separate the song from that scene in my head. Love or hate how Grey's can pluck an unknown musical gem from obscurity and make them the next big sensation, there's no denying that the show knows where to find great music.
What shows have the most notable music for you?