Belle & Sebastian

I adore Belle & Sebastian and even more so in the summer. They produce the most perfect summer tunes. If you don't know their stuff, the indie pop septet could be described as a combination of The Smiths and The Shins, with a dash of Nick Drake and a droplet of Fleet Foxes.
I discovered them when I became obsessed with Teachers, a channel 4 comedy series that started in 2001 and ended in 2004. The series has one of the best soundtracks I've come across (the other programme being Shameless, in which you can enjoy all the best Madchester music). It's still on 4oD if you're interested in watching the four series.
To get to the point, the ending theme of Teachers is 'The Boy With The Arab Strap', the title track of their most successful album. The song also featured in 500 days of summer.

Here is their performance of it on Jools Holland:

Two Belle & Sebastian songs also appeared on the soundtrack of Juno (which is stunning).

'Expectations'

'Piazza, New York Catcher'