I don't even want to know how many times I have written something like this:

var p = db.Products.FirstOrDefault(x => x.Id == someId);

Then 4 lines down, do it again almost exactly the same, over and over, at least once for each table. Well... no more! Now that I am able to get the Primary Key of any Linq-to-SQL talbe, it is trival to be able to write a universal get statement so I can simply do this when I want to grab an object by it's Primary Key:

Product p = db.Get<Product>(someId);

What if I want to get an item that has a Guid as a primary key? Same thing, it doesn't matter:

Guid gId = 
  new Guid("4fcc0b82-b137-4e4b-935e-872ed662ba53");
Gizmo g = db.Get<Gizmo>(gId);

If you you give the wrong Type of Key, it will tell you in a nice ArgumentException:

Gizmo g = db.Get<Gizmo>(5);

Error:

Primary Key of Table and primaryKey argument are not of the same Type; Primary Key of Table is of Type: System.Guid, primaryKey argument supplied is of Type: System.Int32

Here is the code without any error handling:

public static T Get<T>(this DataContext dataContext,
  object primaryKey) 
    where T : class, INotifyPropertyChanged
{
  return dataContext.GetTable(typeof(T))
    .Cast<T>()
    .Where(GetPrimaryKey<T>()
    .Name + ".Equals(@0)", primaryKey)
    .FirstOrDefault();
}

The full code is available in my Utilities class on CodePlex. This requires System.Linq.Dynamic.