Of course almost all image editors have the abilities to create thumbnail image, but I am talking about software that is specially designed for creating thumbnail images.
Quick Thumbnail is an online thumbnail creator. All you need to do is go to this website, upload an images and select the format you wish to create.
You can choose to resize by percentage, fixed size or choose from a list of standard size.
Once the image have been resized, just rightclick on the image and download it to you computer.
KD thumbnail is a freeware, it can create multiple thumbnails at one time. The user interface is quite straight forward. It has the abilities to rename the resized files by adding prefix or suffix.
Easy Thumbnail is another freeware for creating thumbnail. As the name denoted , it is very easy to use.