Free Online City Guides
The German airline Lufthansa offers useful and free guides to most world cities at http://travelguide.lufthansa.com. They don't include detailed information on hotels, which are usually the biggest cost of travel, so they don't totally replace guidebooks. However, they provide a lot of information on topics such as how to fit in, maps, weather and sometimes attractions.
For example, the guide to Vienna has a list of the top ten attractions in that city, with details on opening hours and a link to the Website of each site, so you can check the entrance price. Not every guide includes this feature, perhaps because attractions of the conventional sort are relatively limited in some cities.
I checked the guides for two cities I visited on the same trip about a dozen years ago, Vienna and Amman. Vienna has plenty of and world-class palaces and museums such as the Hofburg and Schoenbrunn Palaces, the Kunsthistorisches and Belvedere Museums, and even a whole museum quarter of the city. The guide lists 10 of these top sights.
Amman is a pleasant city but relatively lacking in must-see places, other than the Roman Theatre downtown and the citadel. Lufthansa's guide to Amman does not include attractions.