Lonely Planet
The one thing you can't travel without..?
my rubber duck, he goes everywhere I do, it's a bit of an obsession now!
Quoting opospa
Lonely Planet
Oh yes, definitely that too! 
A small sleeping bag my mom made me. It's just cotton with no padding so it rolls upp easily and goes well in my backpack.
Can't even count the times that bag has made me comfortable on dirty boats and in infested hostels. Plus, before i had it washed on the way it smelled like home every time i took it out
Also a small diary to keep track of where you've been and when, and who've you met on the way. People and places start melting togeather and it can be fun to remember how you got to where you are.
Usually I take my hammer and sample bags. It doesn't matter where I go I'll still find some interesting geology there.
A good book is essential, usually something difficult which I wouldn't read at any other point other than long bus journeys.
Some excellent advice from you guys
I reckon I'll have to buy a bigger rucksack to take my hairdryer, rubber duck, the wife, GHD's, hammer.......Sticky substances?!!
I'm going to suggest a length of fishing line as a make shift clothes line
Keep them coming 
I love the rubber duck travels, what a great idea!
One or two clean pillow cases. If i'm staying in hostels (not in friends place) i'll put them over the hostel's pillow case
duct tape.....it fixes everything! also handy for taping up packages to send home, when you've accidently bought too much stuff to fit in your backpack! (wrap it onto a pencil to save on space)
and my rubiks cube...such a good conversation starter, esp. if you can complete it! ;o)
Quoting alsalis
my rubber duck, he goes everywhere I do, it's a bit of an obsession now!
Your comment reminded me that we rarely go anywhere without Gnorman & The Gnomads!!!!