'The London Pass' claims to offer good savings. However, you need to spend some very busy days, visiting some of the most expensive places, to get the best value from it:
http://www.londonpass.com/
For travel on a single day, buy a One Day Travelcard for Zones 1 & 2. This costs �6.80 (or just �5.30 if you don't require weekday travel before 0930). It gives you unlimited bus and tube travel within central London and also gets you one third off scheduled riverboat services. (There's also a 3 day Travelcard available, for Zones 1 & 2, but there's not a version that offers 'after 0930 (Mon-Fri)' travel. You have to pay �17.40 for an 'any time' ticket).
If you're staying for a few days, an Oystercard is probably better. You pay �3 to get the card. (That's refundable but most people just keep the card for future visits). Then you 'charge' the card by putting credit onto it (in a similar way to paying money into your bank to make some credit available on your ATM card). The fares you pay by using up the credit on your Oyster card are much lower than cash fares and the maximum that you'll pay each day is capped at 50p lower than if you used a One Day Travelcard for the same journeys.
Chris