Sometimes it's hard to tell if the camera's level, even when it's on a tripod. This is my $1.60 solution to the problem.It's possible to buy a hot-shoe level on eBay for eight or ten bucks (after shipping), but I always prefer to try the super-cheapskate options first.
(The extra-super-cheapskate option is to buy a torpedo level at the dollar store, remove one of the level tubes and use it as-is, and sell the torpedo level at the flea market.)
