How to load plugins in .NET?

As of .NET 3.5 there's a formalized, baked-in way to create and load plugins from a .NET application. It's all in the System.AddIn namespace. For more information you can check out this article on MSDN: Add-ins and Extensibility

The following code snippet (C#) constructs an instance of any concrete classes derived from Base found in class libraries (*.dll) in the application path and stores them in a list.

using System.IO;
using System.Reflection;

List<Base> objects = new List<Base>();
DirectoryInfo dir = new DirectoryInfo(Application.StartupPath);

foreach (FileInfo file in dir.GetFiles("*.dll"))
{
    Assembly assembly = Assembly.LoadFrom(file.FullName);
    foreach (Type type in assembly.GetTypes())
    {
        if (type.IsSubclassOf(typeof(Base)) && type.IsAbstract == false)
        {
            Base b = type.InvokeMember(null,
                                       BindingFlags.CreateInstance,
                                       null, null, null) as Base;
            objects.Add(b);
        }
    }
}

Edit: The classes referred to by Matt are probably a better option in .NET 3.5.