It's hard to say, since you only really remember when you hear it on the radio. But here's a few (they're mainly 90's alt-rock, since that was my growing up era). These are the ones that, when I hear them, I'm taken back to a time and place far in the past, when everything was simple and everything in life seemed so clear-cut. Each of these has special meaning for me, even if I hate the song itself.

Jumper - Third Eye Blind
Poets - Tragically Hip
The Sign - Ace of Base
Old Apartment - Barenaked Ladies
Let Me Go - Cake
Sunshower - Chris Cornell
Karma Police - Radiohead
Lots of Nirvana.. hard to pick one
Freshman - Verve Pipe

And, like everyone else said, much more.