I agree that a dehumidifier is a good idea in general, as long as your
basement stays above 40 or 50 degrees F. If you just want to protect
your tools, though, you could get a little box of silica gel - I bought
one from Cabelas.com that was intented for gun cases - it can be
'recharged' in an oven. This would be good for a relatively small
enclosed space, and I would argue that it would still be effective even
if the case was not completely sealed.
If you're talking about hand tools rather than power, it's a good idea
to wipe them with paste wax or spray them with topcote (or some other
rust preventative that won't leave oil on your wood).
Good luck,
Andy