It depends on your budget then. (More money tends to get you more power!)
If you've got £280 to spare, these will REALLY annoy your neighbours!
http://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/pc-speakers/logitech-z906-5-1-pc-speakers-10725157-pdt.html
£75 will buy you these:
http://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/pc-speakers/logitech-z533-2-1-pc-speakers-white-10139697-pdt.html
£61 still gets you quite a bit of power:
http://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/pc-speakers/logitech-z506-5-1-pc-speakers-09032568-pdt.html
£45 should be enough to fill your living room with sound anyway though:
http://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/pc-speakers/logitech-z333-multimedia-2-1-pc-speakers-10024650-pdt.html
£25 is probably about as low as you can go and still not feel too cheated by the sound output:
http://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/pc-speakers/logitech-z213-2-1-pc-speakers-10024979-pdt.html
Audiophiles might point out that there are better quality products than Logitech ones (etc) available, such as Bose, but:
(a) they cost far, far more ; and
(b) there would be little point in paying for really high quality speakers when you're only feeding them from a laptop (because the sound cards in laptops can't produce 'studio quality' sound anyway).
Just one last thought:
If you want to share the sounds with friends then, of course, you'll need some decent speakers. However if it will only be you listening to the output from your laptop, I'd recommend forgetting about speakers and buying some wireless headphones. (I love mine; I can have music blasting into my ears at 3am, as loud as I like, without my neighbour's complaining!)