I go places.
I do things.
Sometimes I take pictures.
In your wallet
one each of Mastercard and Visa: Mastercard, for shopping and id theft; Visa, for ATMs and cash. Rumor is Capital One doesn't have Foreign Transaction Fees.
A student-ID for places that give discounts
On your person
passport
a nice metal pen like a Cross
As many hidden Benjamins in reserve as you think it would take to make problems end.
Carry-on
a photocopy of your passport and itinerary
Earplugs
an extra pair of contacts
Sudafed/Tylenol PM/general medications in a ziplock and some tissues
iPod and headphones, charger
Mobile phone with wired-handsfree and charger
a thick paperback book, or two.
Pad of paper and pens
a pack of gum to clear that flight-nap taste from your mouth
a couple blank CDs and DVDs so you can pay $0.25 each instead of $10 each.
Or, better, a portable hard drive to back up your photos onto, should the camera be stolen or confiscated.
a ziplock full of Clif Bars or Power Bars or whatever compact energy bar you can tolerate.
Do you need the umbrella? It's bulky, but the Gustbuster can stand up to Irish wind.
Checked
a photocopy of your passport and itinerary
extra contacts
Like Towelie says, always bring a towel
You're unlikely to get deported if you bring along a small knife to go fishing with.
Put your shampoo, saline solution, and toothpaste inside zip lock bags, and then put those inside another plastic bag. They will leak, trust me.
Maybe some sunscreen?
shaving razors
nail clippers if you're traveling for longer than ten days or so
Tide has tea-bag sized sink-washing soap packets, easier than a bag
of powdered detergent.
A small LED flashlight is a Very Good Idea, whether to illuminate dark shadows in ruins or to guide the way as cities undergo their daily power conservation.
What to leave at home:
Yet another photocopy of your passport and itinerary
Do you really want to lose your house and car keys in another country?
Anything else you don't want stolen.
And then relax, because if you forgot something you can just buy it over there.