After growing up with nearly all the music already named in this thread, figured I'd add selections that are more recent (and completely unfitting considering what folks have listed).
-For relaxing:
Stick Figure - Fire on the Horizon; Sound of the Sea
Sublime with Rome - Take It Or Leave It; Wherever You Go
Steam Powered Giraffe - Honeybee
The various mixes made by Dyalla
Lots of Jack Johnson
Mostly anything from ChilledCow or Chillhop
-For something that requires a little more energy:
Mastodon - Toe to Toes; Steambreather
Ghost - Cirice; Absolution; Faith; Square Hammer
Avatar - Statue of the King; Hail the Apocolypse; The King Welcomes You to Avatar Country
I could have gone with Tool or Alice In Chains or numerous other mainstream rock bands but the 3 bands listed above are a lot of fun to listen to, IMO.