I seem to recall once reading that in Weimar Germany, one of the political parties dismissed and replaced its candidate between the two rounds of a presidential election in the 1920s. The candidate didn't die or resign, nor did he become (or be found to be disqualified) -- I think the party simply thought he wasn't popular enough.

We've had situations in Australia (eg 1993 and 2001 federal elections) where candidates died between nomination and polling day. (I'm not sure if voluntary withdrawal is allowed by law: and while candidates are occasionally found to be disqualified, on citizenship or "holding an office of profit" grounds, this usually occurs months after election day). For the House of Reps, with single-member electorates, that election is cancelled and a special poll follows about a month after the original scheduled date. For the Senate, with multi-member proportional representation, the election proceeds as normal and any votes for the dead candidate are immediately re-allocated to the voter's next preference -- usually the next-highest-ranked candidate on the party ticket.