class helper
Class helpers allow you to extend the interface of an object instance without inheriting from it.
TStringsHelper = class helper for TStrings
  function Contains(const Value : string) : boolean;
function TStringsHelper.Contains(const Value : string) : boolean;
  Result := IndexOf(Value) <> -1;
Further examples
David Glassborow discusses the pros and cons of class helpers and gives examples of class helpers for TStream, TCanvas, TList, TDataSet, TField, TTreeNodes and TRect.
David Glassborow provides more example class helpers for TList, TStrings and TStream.
Serg at The Programming Works uses class helpers to work around a bug in TFileStream.
Alex Ciobanu (Emb.) uses class helpers to add Windows 7 taskbar behaviour to TCustomForm.
Jolyon Smith uses class helpers to add client side functionality to remoting library code.
Jeroen Pluimers uses a class helper to obtain the unit name from an EAssertionFailed exception class.
Zarko Gajic extends TStrings to accept data of multiple types using a class helper.
Anders Melander extends TApplication with a class helper to retrieve the application version information.
Primoz Gabrijelcic describes how to add enumerators using class helpers.
Mason Wheeler uses a class helper to add an enumerator to ThreadedQueue<T>
Allen Bauer (Emb.) uses class helpers to allow a TControl to receive the results of an asynchronous task.
Jeroen Plumiers walks on the wild side by combining a class helper for TObject and a "with" statement.
Further reading
Joe White investigates why virtual methods can't be declared inside class methods.
StackOverflow discussion on the uses of class helpers.
The case against class helpers
Jolyon Smith presents a comprehensive argument against the use of class helpers, providing alternative constructs to accomplish a similar goal.
Jolyon Smith continues his arguments against class helpers but describes a safer way to use them.
Jolyon Smith surprises himself by finding a use for class helpers but still disapproves of the feature in general.
Yet more discussion from Jolyon Smith on why class helpers are responsible for all the evil in the world.
Mike Prestwood provides a comprehensive introduction to class helpers with discussions of their pros and cons and recommendations for their use.
Jim McKeeth delivers a CodeRage 2 video presentation on class helpers.
Alistair Christie presents a video introduction to class helpers
Usage within Delphi
Allen Bauer (Emb.) discusses how class helpers were used to produce the non-breaking Delphi 2007 release.
Hallvard Vassbotn discusses the use of class helpers in Delphi 2007 using the example of the Vista glass properties to analyse their usage in great detail.
Mat DeLong reveals a dirty little secret about class helpers. Since D2010 they allow access to private variables.