Definitely recommend Loquax - link above in Casmies post - if you register (free) and no spam - you can use the 'tracker' which helps you keep track of what you've entered and where. Plus there's a very helpful forum if you get stuck on questions, have problems with comps etc etc
Some of the survey sites are ok - but generally only the 'general' surveys that don't ask for specifics about insurance, pets, etc etc -
The following don't seem to send any spam and I know people that have won on them
72Point
NOP surveys (tends to be -ipoints that you win there though)
Tickbox
I'm sure there's a few others - but can't remember then off hand.
I've won quite a few prizes - but not many recently (But I had a LONG break - over 1yr from it hence the lack of prizes). Most recent was a silk bag and scarf worth £70 that I won on the New Women website, past prizes have included a night in a 5 star hotel in Manchester, Dinner, Lunch, Breakfast and Bed all included, plus tickets to watch Man U v Newcastle United (doesn' sound excitign but was great fun actually). It was a 'game' that we had to play and I was only 2 points off the top prize.........which was TWO Nights in the hotel and £1000 as well as VIP match tickets.
The thing is with prizes is that if you don't enter them, you WON'T win. Also don't forget that sometimes the closing dates can be a LONG way in advance, so you may have to wait and find out.