It's called a 'micro-scooter'. One of the most popular outdoor toys at the moment. They tend to come in two sizes, one for kids from about 3 -5 and one for older (5+ kids).
I think you photo is of one of the cheaper replicas (eg from Argos). The genuine article costs about £55 (for the young version) and £95 (ouch!) for the larger version.
We don't stock them at Little Sardines but I actually bought one a few weeks ago in John Lewis for my son's 3rd birthday and he's is flying around on it at every opportunity. I really like the lean left / lean right turning option that the 2 front wheels give. In fact to be honest I was quite tempted to have go myself when he wasn't looking (but I didn't...honest!)