This document is largely inspired by the Ruby Style Guide.

No one man should have all that power.
-- Kanye West

This is an evolving document. Submit a pull request and start the conversation!

Not complicated, it's simple.
-- Big Sean

• Limit lines to 80 characters.

• Use 4-character tabs to indent.

  'Bad
  If blnSomething Then
    Msgbox "True" '<~ 2-character indents
  Else
    Msgbox "False"
  End If
  
  'Good
  If blnSomething Then
      Msgbox "True" '<~ 4-character indents
  Else
      Msgbox "False"
  End If

• All conditionals, loops and blocks should be indented.

  'Bad
  If lngNumber >= 0 Then
  Msgbox "Yep"
  Else
  Msgbox "Nope"
  End
  
  'Good
  If lngNumber >= 0 Then
      Msgbox "Yep"
  Else
      Msgbox "Nope"
  End
  
  'Bad
  For lngIndex = 1 To lngLastRow
  lngCounter = lngCounter + lngIndex
  Next lngIndex
  
  'Good
  For lngIndex = 1 To lngLastRow
      lngCounter = lngCounter + lngIndex
  Next lngIndex
  
  'Bad
  With wksSource
  Set rngSource = .Range(.Cells(1, 1), .Cells(lngLastRow, 1))
  End With
  
  'Good
  With wksSource
      Set rngSource = .Range(.Cells(1, 1), .Cells(lngLastRow, 1))
  End With

• Do not write multiple statements on a single line (using :).

  'Bad
  For i = 1 To 10: DoSomething(i): Next i
  
  'Good
  For i = 1 To 10
      DoSomething(i)
  Next i

Oh that looks like what's-her-name, chances are it's what's-her-name.
-- Drake

• Use Option Explicit to mandate variable declaration.

  'Bad
  Public Sub MyMacro()
      'do something
  End Sub
  
  'Good
  Option Explicit
  Public Sub MyMacro()
      'do something
  End

• Declare variable types explicitly.

  'Bad
  Dim MyNumber
  Dim MyBlock
  Dim MyVariable
  
  'Good
  Dim MyNumber As Long
  Dim MyBlock As Range
  Dim MyVariable As Variant

• When declaring variables on one line, they ALL must be declared explicitly.

'Bad
Dim lngLastRow, lngLastCol As Long '<~ lngLastRow is a Variant, NOT a Long

'Good
Dim lngLastRow As Long, lngLastCol As Long '<~ both variables are Long

• Use a two statement declaration for objects rather than the Dim X As New Y syntax.

'Bad
Dim col As New Collection

'Good
Dim col As Collection
Set col = New Collection

• Prepend all variables with a 3-letter code to indicate its type. This is commonly referred to as Hungarian Notation (or, more accurately, Apps Hungarian)

  Variable Type	3-Letter Code
  Boolean	bln
  ChartObject	cho
  Collection	col
  Comment	cmt
  Date	dat
  Double	dbl
  FileDialog	fdo
  FileSystemObject	fso
  Long	lng
  Object	obj
  Pivot Cache	pvc
  Pivot Table	pvt
  Range	rng
  Shape	shp
  String	str
  TextStream	txs
  Variant	var
  Workbook	wbk
  Worksheet	wks
  WshShell	wsh

  EXCEPTION: input variables to function should NOT have a 3-letter code. These variable types can be identified trivially by Intellisense and should be named to maximize readability:

  

• When working with whole numbers, use Long instead of Integer.

  'Bad
  Dim intValue As Integer
  
  'Good
  Dim lngValue As Long
  
  'Integers are 16-bit and can only store values up to 32,767
  lngValue = 50000 '<~ no issue
  intValue = 50000 '<~ overflow error

  Under the covers, Integer-type variables are converted into Long-type variables, the math is executed, then the Long is converted back to an Integer. Avoid the debugging headache and Dim all integers as Long-type.

• Name local variables in CamelCase. (Keep acronyms like HTTP, RFC and XML uppercase.)

  'Bad
  Dim str_my_variable As String
  Dim strmyvariable As String
  Dim strHttp As String
  
  'Good
  Dim strMyVariable As String
  Dim strHTTP As String

• Name global variables in SCREAMING_SNAKE_CASE. (Keep acronyms like HTTP, RFC and XML uppercase.)

  'Bad
  Dim str_error_message As String
  Dim strErrorMessage As String
  Dim lngCONSTANT As Long
  Dim lngHttpAcceptedCode As Long
  
  'Good
  Dim str_ERROR_MESSAGE As String
  Dim lng_CONSTANT As Long
  Dim lng_HTTP_ACCEPTED_CODE As Long

• Generic 3-letter variable names are OK for common iterators.

  'OK
  Dim wks As Worksheet
  
  For Each wks In ThisWorkbook.Worksheets
      'do stuff to each sheet
      Msgbox (wks.Name)
  Next wks

• Store all global constants in a single unique module named ImportGlobalConstants.

  Option Explicit
  
  Public lng_MAX_NUM_FILES As Long
  Public lng_EXCEL_2003_WIN As Long
  Public lng_EXCEL_2007_WIN As Long
  Public lng_EXCEL_BINARY_WIN As Long
  Public lng_EXCEL_MACRO_ENABLED_WIN As Long
  
  Public Sub ImportGlobalConstants()
      lng_MAX_NUM_FILES = 1000
      lng_EXCEL_2003_WIN = 56
      lng_EXCEL_2007_WIN = 51
      lng_EXCEL_BINARY_WIN = 50
      lng_EXCEL_MACRO_ENABLED_WIN = 52
  End Sub

  This allows you to import all pre-defined constants by adding:

  Call ImportGlobalConstants

  to your script.

• All public, reusable functions and subroutines that are not task-specific should be stored in a unique and easy-to-find module.

  

• Prefer the verb "Create" for functions that return a String.

  Public Function CreateFullFilename() As String
      'the magic happens in here
      CreateFullFilename = ThisWorkbook.Path & "\not_really_this_easy.xlsb"
  End Function

• Prefer the verb "Calculate" for functions that return a Long.

  Public Function CalculateUltimateQuestionOfLife() As Long
      CalculateUltimateQuestionOfLife = 42
  End Function

• Prefer the verb "Get" for functions that return a Workbook.

  Public Function GetExceptionsTemplate() As Workbook
      'the magic happens in here
      Set GetExceptionsTemplate = Workbooks.Open(strFilePathToExceptionsTemplate)
  End Function

• Prefer the verb "Pluck" for functions that return a Range.

  Public Function PluckIYYRow() As Range
      'the magic happens in here
      Set PluckIYYRow = .Range(.Cells(rngIYY.Row, 1), _
                               .Cells(rngIYY.Row, lngLastCol))
  End Function

• Prefer the verb "Collect" for functions that return a Collection.

  Public Function CollectDataFilePaths() As Collection
      'the magic happens in here
      While Len(strFile) > 0
          col.Add (str_GLOBAL_DIR & strFile)
          strFile = Dir
      Wend
      Set CollectDataFilePaths = col
  End Function

• Prefer the verb "Assemble" for functions that return a Scripting.Dictionary.

  Public Function AssemblePairs() As Scripting.Dictionary
      'the magic happens in here
      For lngIdx = 1 To lngLastRow
          dic.Add Key:=.Cells(lngIdx, 1), Item:=.Cells(lngIdx, 2)
      Next lngIdx
      Set AssemblePairs = dic
  End Function

I'mma be fresh as hell if the Feds watching.
-- 2 Chainz & Pharrell

• Close For...Next loops with the iterative variable after Next.

  'Bad
  For Each wks in ThisWorkbook.Worksheets
      'do stuff with each worksheet
  Next
  
  'Good
  For Each wks in ThisWorkbook.Worksheets
      'do stuff with each worksheet
  Next wks

• Prefer With...End With blocks to reduce repetition.

  'Bad
  Set rng = wks.Range(wks.Cells(1, 1), wks.Cells(lngLastRow, 1))
  
  'Good
  With wks
      Set rng = .Range(.Cells(1, 1), .Cells(lngLastRow, 1))
  End With

• Qualify Range objects with a Worksheet.

  'Bad
  Set rng = Range(Cells(1, 1), Cells(lngIdx, 1))
  
  'Good
  Set wks = ThisWorkbook.Worksheets("Data")
  With wks
      Set rng = .Range(.Cells(1, 1), .Cells(lngIdx, 1))
  End With