Steve was right - look at machines in the class "electric die
grinders".

Also you might consider the Rotozip rotary drill-saw. It takes 1/4 and
smaller collets and has a lot of power. It works nicely with an
inexpensive AC speed controller.

There are some small edge finishing machines for formica counters that
look like small routers - and of course take 1/4 shanks. These tend to
be pretty powerful, too.

But if you need the flexible shaft, it's hard to beat the foredom.
It's not a high speed grinder, but it has the torque.
-Dion