One site, www.getyourguide.com, based in Switzerland, has an on-line booking and payment system and definitely has a quite substantial offer: there are more than 700 tours offered in Italy (nearly 200 in Rome alone). In Europe, Italy has by far the largest offer on the site. Spain, Germany and France offer around 300 tours, followed by the United Kingdom, Turkey, Portugal, Austria and Greece. The biggest offer comes from the USA with more than 900 tours offered.
Pricing varies depending on the offer but tends to be relatively aligned: for a 3 hours in the Vatican Museums and Church the cost per person is around 30 to 40 euros (tickets not included). The site is organized like any travel booking site with as much services and content available on the home page, but it is still easy to find your way. The upper part of the site is occupied by revolving photos major destinations.
Another web site that offers similar services is www.excursiopedia.com, which offers a lower number of tours, but is based on a similar business model. The site was launched in 2010 and has a more luxurious look and feel. The home page looks less crowded and the front is occupied by a world atlas where you can choose the continent and final destination to see what is available. The tours offered on the site (or at least those that are featured on the front page) are in general more exotic: a safari on the silk road, a driving a Ferrari in Barcelona or a photo tour in Paris.
Another interesting offer is developed by a recent start-up, www.myguidie.com, based in Poland. The site is still incomplete but has nice ands playful design. On-line booking is not yet available. Services are offered by either official licensed guides, which can charge for their services, or local volunteers who want to share the experience of their city with the visitors and find a new friend or offer a room to rent.
All web sites gather local resources to develop their offer. Their objective is not to develop content (although excursiopedia gives the opportunity to create blog sections and shows some, limited, content).