Betfair Sportsbook provides markets for more sports than the exchange and has a greater number of extra markets for most as well.
The football markets are one of the main areas Betfair turn their attention to and they cover all the main leagues from around the World. Particular attention is paid to UK markets though and they provide a vast number of options per game. A novel feature is the “odds on that” concept where you can Tweet them with your predictions which may not currently be available, such as “Over 4.5 cards, Over 2,5 goals and Over 9 Corners” and they will put it on the website with a price. Your Tweets to them should have the hashtag #OddsOnThat.
Horse Racing markets are also well covered and they provide not just great odds but one of the best-designed betting areas and a huge range of information and race data to help with your selections. They also have a live stream of every race which you have bet on.
Other sports are well covered but not quite a swell as the main ones. However, if you are into Tennis they have odds not just on the match result but you can bet set by set and game by game. Basketball and Cricket also should be mentioned as having many options.