How often has Congress overridden a President's veto?

Here's a document which attempts to tabulate Congressional overrides, arriving at a total of 106 by the end of the Clinton era.

When was the last time it has happened?

The twenty-year rule applies there; it's recent.

Are there any such laws that are still in effect?

At least some are, yes. It would be quite the task to track down all of them and see if they've been superseded in part or in whole, but at least some are still on the books.

One particularly important example would be the War Powers Act of 1973, which was vetoed by Nixon and later overridden. (Nixon also suffered an override when he vetoed the Clean Water Act—but the Clean Water Act has been amended a few times since; hence what I was saying about being partially superseded.)