This new website from charity The City Bridge Trust pulls together most of the parks and gardens of inner London into one convenient map. The site claims to contain over 1,000 parks, and it certainly provides decent, if not quite comprehensive, coverage for zones 1 and 2. Each pinpoint brings up information about the park, including the main features, history, facilities, photos and upcoming events. The site's creators want to get the public involved, and have included sections where community groups can upload event info, or leave comments on a particular park.
Although a website rather than an app, it's all optimised for mobile devices, and is designed to be used while out and about. All pretty neat, but there are also several serious flaws. First, the search doesn't work. Choosing category 'picnic spot' in 'Camden', for example, offers up 'The Rookery' in Streatham as one of only two results (Hello! Primrose Hill?). Second, the site makes heavy use of geotagged images from Flickr, but provides no link or attribution. In other words, it's full of uncredited images. Although the site's not-for-profit, this is still likely to nark some people off.
In summary: a handy way to uncover parks and gardens you weren't aware of, but needs a few serious tweaks if it's going to thrive.