If you're doing it for the acid (i.e. to cut the heat in a spicy dish), you might try cream of tartar if you have it lying around. I've never actually tried to substitute tartar for lemon juice, but lemon juice is the most common substitution for cream of tartar, so it stands to reason that it works both ways. (Note: You would use about 1/3 as much cream of tartar as the amount of lemon juice that's called for).
If it's for general flavouring, this may sound insane, but if you happen to have any cherry brandy or even regular brandy lying around, try that. A solution of sherry and cider vinegar is an OK substitute but doesn't quite have the tartness and strength of lemon juice. While cherry brandy obviously doesn't taste the same as lemon juice, it's often just as good or better in recipes that call for it.