sockmuppet.com

• Amazon (UK)
  
• CD-WOW
• Thinkgeek (NB: Beware of customs charges if you're in the UK)
• Expansys
• Maplin
• Empire Direct (In fact, they have pissed me off, but they sell cheap stuff.)
• Crucial
• LinITX
• Play.com
• Netshop
• EasyArt
• Jessops
• eBuyer (Only used once, but seem ok)
• Komplett (Recommended to me, but I haven't used them yet)