For the sad times, Dry Your Eyes by The Streets.  It captures the moment of being dumped so perfectly that I choke up when I hear it:
So then I move my hand up from down by my side 
It's shakin’, my life is crashin’ before my eyes 
Turn the palm of my hand up to face the skies 
Touch the bottom of her chin and let out a sigh 
‘Cause I can’t imagine my life without you and me 
There’s things I can’t imagine doin’, things I can’t imagine seein’ 
It weren't supposed to be easy, surely 
Please, please, I beg you please 
She brings her hands up towards where my hands rested 
She wraps her fingers round mine with the softness she’s blessed with 
She peels away my fingers, looks at me and then gestures 
By pushin’ my hand away to my chest, from hers 
For optimism, Drive by Incubus.  It also reminds me of a special woman in my life.
It's driven me before, it seems to be the way 
That everyone else get around 
Lately, I'm beginning to find that when I drive myself, my light is found 
Whatever tomorrow brings, I'll be there 
With open arms and open eyes yeah