The Most reusable code in my libraries

• First and foremost I think is the Member class  
• No more binding using brittle string literals. 
• Compile time member access name safety.
• BDictionary
• Allows for reference of a an item by key or by the index
• String Transformations
  
• Can take comma separated values,CR/LF, excel copied, SSMS results column copied  and transform them into
• a SQL in clause
• an array initializer
• a string array initializer
• bThread(of T)
  
• Threading implementation with strongly typed result value
• Wraps processing in try/catch so the thread doesn't throw an exception
• bMacros - originally named bVbMacros but the snippet section does C# now
• takes a partially formatted string and creates a code snippet for use in Visual Studio
• Takes a VB object constructor and automatically generates backing fields and public readonly properties to expose those values
• DebuggerWriter:System.IO.TextWriter
• Wrapper to allow you to stream output to the Debugger
• XmlFont
• Makes serializing a System.Drawing.Font simple
• Windows Forms specific:
• TextBoxWriter:System.IO.TextWriter
• Wrapper to allow you to stream output to a WinForms Textbox
• Control Extensions
• Extension methods for thread safe operations on the UI
• Extension methods for nullable value binding
• An extension for populating a comboBox with an enum Type's values
• ErrorProviderManager
• Wrapper for the ErrorProvider class
  
• Centralized repository that takes a control, a Func delegate and the associated error message.
• hooks into the control's validating event to run all the Func associated error conditions, if it fails validation, the ErrorProvider is updated accordingly, and vice versa

That's a fairly small amount of really reusable code. To me, whether or not the reusable code I wrote gets reused much is important, but so is what I learned from aiming for framework-worthy code and learning from the failures.