1. A travel adaptor that looks like this:
In hostels where plug sockets are already limited, sticking this massive thing in the way of 2 sockets is a sure fire way to annoy the hell outta people. Instead buy a standard single adaptor or a global adaptor that looks something like this:….
2. Any smart clothing:
You really don’t need it; guys take one shirt, girls take a dress which you can dress up for going out or down for the beach. Although it seems natural to have your favourite pair of jeans, in a lot of places these aren’t necessary either. Think about where you’re going and the lowest temperature likely. Its much better to take a pair of linen trousers or chinos/combats for boys, they are lighter, less bulky and dry quicker if they get wet.

3. Tonnes of toiletries and medical supplies.
Let’s face it unless its prescription you’ll be able to get it out there, although an essential kit of a couple of plasters, painkiller etc. is very useful you really don’t need flu meds or Gaviscon or anything you don’t use regularly.
4. Lots of shoes
You only need 2 pairs of shoes really for most regular backpacking. Trainers and flip flops. If you’re really keen to take more footwear maybe chuck a pair of plimsolls/espadrilles in there.

If you have the space and don’t mind lugging them about great, but in most cases you’ll get bored of this pretty quickly and end up ditching them. Best option is to photocopy anything you really feel you need, then you can throw it away once you’ve moved on, or do your research locally in hostels etc (many hostels have resident copies of lonely planet at their reception which you can borrow) failing this jump on the internet.