Ah, there is some great songs being posted which I think will help others, well done up to now guys.
Whenever I feel down, there are a couple of songs that really make me think and give me hope:
Something inside so strong - Labi Siffre (I don't know if anyone has already posted that, but it's a really inspirational song, well, for me it is.)
Everybody hurts - REM, Is another song that gives me hope. The lyrics just listening to them, makes me feel less alone and that everybody goes through some sort of pain at one point in their life.
I'll definitely add to this list at a later date when I can remember some more songs that provide hope. That's all I can think of right now!