My friends used to laugh at me, until someone asked what I carried in it. I put the pack on the table and pulled out:

Book I was currently reading
Cell phone
Umbrella (very useful, as I live in Seattle)
Small towel (I read the Hitchhiker's Guide, too)
PDA (and contribute to the HHG on occasion)
Digital camera
Spiral-bound notebook
Pen
Small first aid kit
Tin of Altoids
Two condoms (you never know when...)
About $3 US in change

Oddly enough, I usually keep my wallet and keys in my pockets. If/when I get a sporran, those are probably all I will keep in it.