Music and other CDs; DVDs; Blu-Ray
Good back catalogue and non-chart bargains. Links to other suppliers of new and used titles. UK delivery free if Amazon are supplying direct.
CDs, DVDs & Blu-Rays. Cheaper than some but not usually the cheapest, typically £8.97 for chart CDs. Free delivery on CDs and DVDs. 
Much wider selection of DVDs and CDs than in their shops. Not the best prices on most chart titles, but good special offers. Free delivery on CDs and DVDs.
Not usually best value for chart CDs but always some excellent special offers. Free standard UK post.
CDs, DVDs and games mostly at relatively modest discounts but a few at really good prices. Worth looking for less common titles. Post free in the UK
CDs, DVDs, games and gadgets. Also a few cameras, books, TVs etc. Free delivery.
Specialist entertainment, including aviation, maritime, military, transport, sport, heritage and others. Free delivery over £30.
Woolworths is back! The product range is much reduced and now focuses on toys, kidswear, entertainment and party. Free UK delivery on entertainment.
Once Virgin Megastores, now part of the Hut group. Excellent deals on DVDs, games, music and books. Free delivery to the UK on everythingLook on this page if you want CDs or DVDs to buy. There are separate pages for DVD rentals and downloads.
Most of the online stores have chart CDs in stock at promotional prices. By and large, the best prices for chart CDs that are available internationally are offered by the mail-only megastores.
CDWOW used to be amongst the cheapest and is probably the best known supplier of discount chart CDs but they have recently been taken over and their prices are not as cheap as they used to be.
These online stores source their CDs internationally so albums that are only released in the UK can be quite a lot more expensive. For UK-only releases, stores based in the Channel Islands are often best value.
The hut and Play.com tend to have more non-chart titles in stock or available at short notice, even back-catalogue titles and albums that you didn't know made it to CD.
If Play.Com don't have a title in stock themselves, their web site finds others that might. Similarly, Amazon have a massive selection themselves but also refer you to new and used versions available elsewhere. Their chart album prices are certainly beatable but they have some bargains in titles that were released less recently, as well as in compilations and boxed sets.
HMV are particularly good at major offers on a narrow range of promotions, especially boxed sets. Prices on their web site are usually lower than their shops and as they deliver free, you are generally better off ordering from them online than going to a store.
If you can't wait for the post, take a look at the downloads page for a choice of ways to download your music and many films immediately.
For DVDs, HMV have really good offers on boxed sets and a narrow range of other titles and their online prices are usually better than their store prices. With free delivery as well, it is cheaper to buy online from HMV. Play.com also have excellent offers on selected DVD titles including some major films.