10x developer is very rare, but I have worked with guys like that - though of course, this depends on tasks. If task is well defined and bounded by a given tool set and rigid rulesets, etc, 10x cannot really be achieved. On the other hand, if there is just a vision of achieving goal X, and the median developer doesn't even know what tools to start with, an outstanding developer is much more than 10x productive than median.

However, in regular development jobs, I have also seen 0.1x developers, and worse. Even guys who definitely do damage if they are given access to the tools. The variation is huge.