Here is a collection of my favourite homemade extension methods. Most have been included in the plain framework.
#1: My extension method for making !string.IsNullOrEmpty(somestring) more readable.
#2: Bringing the IN operator from sql to .net. It's implemented on enums but could be any type.
#3: Execute an operation on a collection. I use this all the time and I also have a version that can return the result of an operation as a new collection. That is basically equal to what the .Select extension from System.Linq does. The functional approach of F# inspired me to this.
#4: Get a typesafe value from a web request
#5: Gets all the controls of a certain type from a form. Originally I also had a version that operated on any type of object. I soon found out that searching an object structure without any constraints is a performance disaster. Being limited to finding controls on a form is much better as it's a well defined hierarchy that easy and fast to traverse. The idea springs from looking at JQuery. Some examples of usage can be found in my blogspot about oneliners in .net
#6 Save an object. This operates on types not defined yet. If you implement the IBusinessEntity<> interface you will automatically get a Save() method. It gives a feeling and ease of use as that of the Active Record pattern, without the object knowing anything about persistence. The plain framework also contains extension methods for Delete() and Validate().