Good level format

Need more details. 3D or 2D? Tile based or screen-based (ala Baldur's Gate)? Isometric? How many layers? Are objects free-rendered onto a tiled background, or are they turned into tiles themselves?

If you've defined the data structure for your world, the file format to store it in should be pretty easily apparent, unless it's an extremely complex data structure.

~Pax

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Sorry for the lack of data:
I'm making the program/game in 3D using OpenGL and i'm making it screen-based.
The objects' info should be loaded from the map/level and each object should be rendered separately from the map/level.
Thank you for your help!
PEACE!.

The Aurora Engine (NeverWinter Nights) uses a tile based map format, basically , what it is is a collection of square chunks which can be interconected to one another, this should not be confused with old 2D Isometric tiles, where a tile represented a square meter, instead each tile may cover several square metters.

On this system, the player entity and other NPC's are bound to the ground (no jumping, no overhangs, no going under or over [pick one] bridges), collision is done using a collision mesh, which is a low level detail copy of the tile, the height of the entities over the ground is calculating by casting a ray directly under its position and finding where does it intersect the collision mesh, this is enought for an RPG.

If you want gravity, jumping, physics, going thru overhangs and other stuff, its better to use convex shapes for collisions, for which the BSP format excels, so I'd recomend taking the text based MAP format and build uppon it, you can mix a heightmap for outdoor terrain and make edifications with the map format, then place your edifications over the terrain, this is what Torque does.

Hope that helps.

Ok, let me add a SMALL info:
I'd prefer if the engine was free, since i cant buy anything using the net.
I dont know if the Aurora Engine is free, if it isnt, thanks for the tip anyway.
PEACE!

Are you looking for an engine or a file format specification? because the file format spec you can create yourself without paying any royalties.

There is some documentation at http://nwn.bioware.com/builders/ on how to build tiles and such, NeverWinter Nights was meant to be highly mod friendly (though that's arguable), so you could get a copy and mess with that.