I just returned from a 2 month solo trip around Europe. I carried only my pack and a small daybag with me. Being a "girly-girl" I tend to over pack, so here are the things I should have left home:

Rain jacket – I never used it, but it is good to have
High heel sandals – I wore them once to a club and ended up leaving them behind in Spain
Make-up – I put on make-up 3 or 4 times… when it is 100+ degrees make-up just melts off your face! Also, backpacking isn’t a beauty contest, and most people aren’t putting on make-up and doing their hair in the hostels.

I also brought too many clothes… I actually didn’t wear 2 of the shirts I brought, and I never wore the dress I brought to wear out to clubs and stuff like that.
I didn’t bring my hairdryer, and after a few days of worrying about my hair, I began to love the time I saved each day not drying my hair. Sure, my hair wasn’t perfect, but hey, I was backpacking!!

What came in useful…
Swiss army knife with corkscrew – I used it every day for cutting bread, cheese, fruit and opening wine
Wet wipes – great for cleaning up on train rides and hot days
Sarong – I bought a sarong in Cinque Terre and don’t know how I made it halfway through my trip without it. It can be a towel, a blanket, a skirt, you can wear it from the shower… It has a million uses
Flashlight – great for when you come in late and dont want to wake up everyone in your room
Alarm clock – I would have never made those early morning trains without it

Trust me, pack less than you think you will need. Europe does have stores, and you can buy anything and everything there!!