When I was working the update for my Ajax demo, I needed to create thumb nail from a director of photos.  There are tons of tools out there to do this, but I thought I'd share the very simple code I used. 

It takes all the jpgs in the root path and creates 160x120 thumbnails of them. It also copies the original photo into fullpath. 

namespace ThumbNailer
{
    class Program
    {
        static void Main(string[] args)
        {
            string rootPath = @"C:\Users\brada\Desktop\ForDemo";
            string thumbPath = Path.Combine(rootPath, "Thumb");
            if (Directory.Exists(thumbPath)) DirectoryDelete(thumbPath);
            Directory.CreateDirectory(thumbPath);

            int imageNumber = 0;
            foreach (string s in Directory.GetFiles(rootPath, "*.jpg"))
            {
                imageNumber++;
                Console.WriteLine("{0}:{1}", imageNumber, s);
                Image i = Image.FromFile(s);
                Image thumb = i.GetThumbnailImage(160, 120, null, IntPtr.Zero);
                thumb.Save(Path.Combine(thumbPath, GetName(imageNumber)));
            }
        }

        static void DirectoryDelete(string directoryName)
        {
            foreach (string filename in Directory.GetFiles(directoryName))
            {
                File.Delete(filename);
            }
            Directory.Delete(directoryName);
        }
        static string GetName(int imageNumber)
        {
            return String.Format("{0}.jpg", imageNumber);
        }
    }
}

Update:  A couple of folks asked me about how to do this in ASP.NET... Bertrand has a much more complete example of that here:

http://dotnetslackers.com/articles/aspnet/Generating-Image-Thumbnails-in-ASP-NET.aspx

http://weblogs.asp.net/bleroy/archive/2007/12/05/what-interpolationmode-and-compositingquality-to-use-when-generating-thumbnails-via-system-drawing.aspx