NSMB Level Format Documentation
Written by Treeki. Copyright 2007
All data (except where stated) here was discovered by me (Treeki).

BGDAT files:
10 bytes for each object.
A map:
2 = Tile type
2 = X
2 = Y
2 = Width
2 = Height
Each of these are encoded in 2 bytes.

Formula to convert 2 byte format to an integer:
((second byte * 256) + first byte)

Formula to convert integer to 2 byte format:
first byte = number mod 256
second byte = (number - (number mod 256)) / 256

All positions are in tiles, not in pixels. Each tile is 16x16 pixels.
Collisions are determined by the tile; you do not have to set them yourself.

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Normal level files:
-- To be updated as more is discovered --

These values are encoded using the same 2 byte format described above:
0x74 = Time Limit
0x76 = Background Graphics Set
0x7C = Level Tileset Graphics Set (also, collision data)
0x82 = Background (Near Layer) Graphics Set
0xAA = Background Tile Layout (seems to also change some tiles like question block and brick to their appropiate level's palette)
0xAC = Background Palette
0xC0 = Level Tileset Palette
0xD2 = Background (Near Layer) Tile Layout
0xD4 = Background (Near Layer) Palette
0xDA = ??
0xDC = ??
0xE4 = Start position X? (div. by 0x10 to get real X)
0xE6 = Start position Y? (div. by 0x10 to get real Y)
0xF2 = ?? (I think it changes Mario's pose on starting. 00 80 is normal, 08 00 is groundpounding like in 1-2.)

Enemies/sprite commands are shown this way:
0x30 = Offset at which the first enemy is located

At the offset specified at 0x30, each enemy/sprite command is 12 bytes, as in:
2 = Enemy type
2 = X position
2 = Y position
6 = Additional enemy/sprite command-specific data

Enemy-specific data:
Koopa Troopa/Paratroopa colours: (Thanks to Tanks)
The 4th byte in "Additional data" can be changed as in:
00 = Green
01 = Red
02 = Blue
03 and over = Blue

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TSA Data: (\bg_pnl files)
-- To be updated as more is discovered --

8 bytes for each TSA entry.
Map:
1, 3, 5, 7 = Tile numbers (top left, top right, bottom left, bottom right respectively)
2, 4, 6, 8 = Other bytes (one for each tile number)

Bit map for other bytes: (INCOMPLETE, I'm not entirely sure about these)
7 = ??
6 = ??
5 = ??
4 = ??
3 = ??
2 = Flip on X axis
1 = Flip on Y axis?
0 = ??