it depends, i have 2 saddle bags, a big one for solo rides and a small one for group rides. if i'm going on a really long ride, like say a century or metric century, i'll bring the big one just in case anything happens... twice...

for solo rides:
2 Bontrager Superlight tubes
4 co2 cartridges
1 inflator
2 tire levers
1 mini tool
1 $20 bill
my cellular phone

for group rides:
1 Bontrager Superlight tube
1 co2 cartridge
1 inflator
2 tire levers
1 $20 bill

i don't bring much with me if i'm riding with other people, because i know everyone's willing to help each other out if anything arises. the only reason i carry a lot with me if i ride by myself is because i usually ride before work, and i don't want to have to tell my boss i was a few hours late because i got 2 flats, and i had to walk X amount of miles in cleats...