Meh, Hadoop is much older than MongoDB, is an offline system, and doesn't destroy your data. There is also not much in the way of alternatives. I'm skeptical we will see many "we're switching off of Hadoop" posts anytime soon. That said, there is an growing undercurrent in the Clojure community to roll your own map reduce system instead of using Hadoop.
There's quite a bit in the way of alternatives. Sector/Sphere, Xgrid, and PVM come to mind. Sector/Sphere is perhaps the most direct alternative since it comes with its own distributed filesystem, but if you don't need a full-fledged DFS then the others are worth looking at too.
The main thing that really distinguishes Hadoop is that it's built to only do one kind of distributed processing. In exchange for asking you to don that straitjacket it offers ease of use. However, if your problem isn't naturally a map/reduce problem, or if your main performance bottleneck isn't disk I/O, then the alternatives become a lot more attractive.
