Well depending on where you want to go and what you wanna see there are so many tour comapnies out there
Busabout is a great one if you want to organise everything yourself - they jus supply the travel means, and you do the rest.
http://www.busabout.com/
Or group travel where accomm, travel, and some activites are paid for - there is Top Deck http://www.topdecktours.co.uk/content.asp?Document_ID=16
They are catered for 18-35 year olds and I have travelled with them before and had a good time - well organised and fun
Um, there is Gap Adventures, http://www.gapadventures.com/ I havent used these guys before, but will be when i go to vietnam later in the year, my friend works for them so highly recommends them
I find the shorter trips seem to be more pricey and ya dont get as much value for money as you do the longer ones....
Hope that helps :)