Moved here from General. There have been several discussions on this matter previously - so you can use the Search function to read those threads.
I'll let this thread run simply because you asked for websites that review portable MP3 players. CNET.com has regular features/reviews of MP3 players although some of their editors are biased towards the iPod while others are hell bent upon pushing alternatives down your throat. You need to be an informed reader while reading CNET reviews
As far as I know, THG do review MP3 players. Check out
MobilityGuru - THG's website for everything mobile.