GWC

@0000:
  ; Signature:
  BYTE  0x02
  BYTE  0x0a
  BYTE  "CART", 0x00

@0007:
  ; Number of objects ("media files") in cartridge:
  USHORT  number_of_objects
 
@0009:
  ; references to individual objects in cartridge.
  ; there is exactly [number_of_objects] blocks like this:
  repeat  times {
    USHORT  object_id  ; distinct ID for each object, duplicates are forbidden
    LONG  address      ; address of object in GWC file
  }
 
@xxxx:
  LONG  header_length  ; length of information header (following block):
  struct {
      DOUBLE  Latitude                     ; N+/S-
      DOUBLE  Longitude                    ; E+/W-
      DOUBLE  Altitude                     ; (see url 2)
 
      LONG  unknown_value
      LONG  unknown_value
 
      ; media-ID of icon and splashscreen:
      SHORT  ID_of_splashscreen            ; -1 = without splashscreen/poster
      SHORT  ID_of_small_icon              ; -1 = without icon
 
      ASCIIZ  type_of_cartridge            ; "Tour guide", "Wherigo cache", etc.
      ASCIIZ  PlayerName                   ; name of player who downloaded this cartridge
 
      LONG  unknown_value
      LONG  unknown_value
 
      ASCIIZ  CartridgeName                ; "Name of this cartridge"
      ASCIIZ  CartridgeGUID
      ASCIIZ  CartridgeDescription         ; "this is a sample cartridge"
      ASCIIZ  StartingLocationDescription  ; "nice parking"
      ASCIIZ  Version                      ; "1.2"
      ASCIIZ  Author
      ASCIIZ  Company
      ASCIIZ  RecommendedDevice            ; "Garmin Colorado", "Windows PPC", etc.
      
      LONG  unknown_value
 
      ASCIIZ  CompletionCode               ; only the first 15 chars are needed
  } header
 
@xxxx + header_length + 4:
  ; here should be first object, but it is not important, as all objects have
  ; it's address (offset in .gwc file) known, including the first one:
 
 
@address_of_FIRST_object (with ID=0):
  ; always Lua bytecode
  LONG  length 
  BYTE[length]  content_of_object  ; Lua bytecode
 
@address_of_ALL_OTHER_objects (with ID > 0):
  BYTE  valid_object
  if (valid_object == 0) {
    ; when valid_object == 0, it means that object is DELETED and does not exist in cartridge.
    ; nothing else follows.
  } else {
    LONG  object_type  ; -1=deleted, 1=bmp, 2=png, 3=jpg, 4=gif, 17=wav, 18=mp3, 19=fdl, other values have unknown meaning
    LONG  length
    BYTE[length]  content_of_object
  }
; ============================================================================
; And that is all.
; ============================================================================
 
BYTE = unsigned char (1 byte)
SHORT = signed short (2 bytes)
USHORT = unsigned short (2 bytes)
LONG = signed long (4 bytes)
ULONG = unsigned long (4 bytes)
DOUBLE = double-precision floating point number (8 bytes)
ASCIIZ = zero-terminated string ("hello world!", 0x00)

Sources: 
 - (1) http://ati.land.cz/gps/wherigo/gwc.htm (site down)
 - (2) http://github.com/wijnen/python-wherigo/blob/master/wherigo.py