I saw a couple projects building 3D maps using the default RMXP editor and thought it was pretty cool, but most of them were discontinued.
They weren't too simple to use either and were pretty advanced.

While going through some japanese sites I came across this game

http://www.free-gameland.com/free_kobetu.php?eid=00078

It also uses the default map editor to build a 3D map, and thought it was also interesting.
Though, it is pretty simplistic, using 10 pictures to put together each scene (since it's 1st-person 4-directional)

Well for first person like this one, there is the First Person Labirynth Explorer from Mgc. http://www.save-point.org/showthread.php?tid=422
Beside this one the only other "3D like" system usabe is the H-Mode7, also from MGC. http://www.save-point.org/showthread.php?tid=3151

There's nothing better and usable than these 2 in terms of "3D"

Ya, I was mainly looking for a first-person view so mgc's looks like it would do the trick.
Though I was reading through the mode7 and it seemed kind of difficult to set up.

Maybe "3D" isn't a very accurate term to describe 1st person perspectives.

Looks like he's made a lot of different scripts.

Did you dowloaded the FLPE demo at last?
It has nothing to do with mode 7, the maping is incredible easy, you just set the textures on the tileset, and on map, the 1st layer is the floor, the 2nd are the walls, and the 3rd the ceiling.

If you are serious that you want a 3D game, I have to say that you shouldn't be using RMXP for it since it is made and optimized to make 2D games. Even if it is possible to make 3D games in RMXP, you can't really do much whith it. But there are other free game engines out there that is done to make 3D games in. Unity3D is one of them. It's free if you want basic things, but for reflections and such you need to buy it. But if you acctualy considered using RMXP for it, Unity will be a dream.

@stripe103
As it was stated what he wants it a first person exploring, and FLPE do it perfectly and easly, it's not a full 3D, it's just a 4 directional First Person Labirynth exploring, like the ones in Phantasy Star and SNES Shin Megami Tensei.