Junon in FF7. Hit that alarm and you can level up like crazy early in the game.

Outside Fabul in FF4. This is where I kill everyaone except Rydia off and level her up on bombs until she learns Meteo (well I did that once, but otherwise it's just a good place to bump up the levels a bit).

I'm sure I have lots more, but those are ones that come to mind immediately.

Here's a one that I don't particularly care for as a bonus. In Suikoden 3 there's a trick where you can keep gaining levels by winning a war battle and then dying. It takes a lot of patience, especially since winning the battle can come down to random chance a lot (you don't have to win it to proceed) before you've stacked up several levels.