TileSetMaker

Being a game designer is a satisfactory job most of the times, but it can prove to be quite challenging if you lack appropriate tools.
Fortunately, if you need specific software solutions, nowadays it is possible to choose from a wide variety of applications, such as TileSetMaker.

Simple user interface with no hotkey support

This application comes with a plain design that integrates most of its core functions as buttons while fewer of them are organized in menus. Its features are rather intuitive, as they provide you with tooltips upon hovering your mouse pointer over them.
However, relying on standard keyboard shortcuts such as Ctrl + C or Ctrl + Z is not supported, although the application features the corresponding functions. Therefore, editing content might be quite tedious, as you need to select the desired functions by clicking them with your mouse.

Divide your images into tiles and save them to a library

You can import any PNG, JPG, or BMP file to the application and turn it into tile sets that you can easily implement in your game design projects.
It is possible to either convert the entire document or create a selection and turn it into tiles. Dividing any content into tiles automatically adds the resulting items to a library that you can easily access. Additionally, the libraries can be saved on your computer and opened later, if needed.

Multi-tab editing functions and canvas resizing capabilities

Aside from the features mentioned above, this application also allows you to edit your image files by opening them in multiple tabs. Copying and pasting might prove to be challenging, as it is quite counter-intuitive. The application does not automatically paste the content of your clipboard over the selection you defined but requires you to click the desired area.
To sum it up, TileSetMaker is a lightweight application that can help you create tile sets for your game design projects. However, note that it does not provide you with hotkey support and some of its functions might be quite difficult to use.