Icons8 Lunacy

If you're a web or graphics designer and need a quick, convenient way to view SKETCH files on your computer, you might as well try specialized apps like Icons8 Lunacy.
This application provides you with just enough functionality to help you open SKETCH files without altering the contents in any possible way.

SKETCH file viewer

You can turn to this program if you need a quick way of viewing SKETCH files on your computer. Note that modifying the contents of your documents won't be possible, as this utility only serves as a viewer.
In order to open a file, you need to use the traditional menu in the main screen, as this app doesn't come with toolbar buttons. It is also impossible to open a SKETCH file by dragging and dropping it on the window.

Export SKETCH files

The "File" menu enables you to open and close SKETCH files freely. However, it provides you with additional functions. One of them helps you export the document to PNG format, so that you can use it with other applications. Another one lets you associate SKETCH files with this app, so that you can open them by double-clicking them.
It is also possible to view a list of recently opened items in the same menu. Clicking any of them opens them instantly, if they're still available in the recorded location.

Additional info

After you open a sketch file, you can view additional data about each of its components by simply clicking them. The details will be displayed in the far right part of the screen.
You can view info such as position, rotation, opacity, fills and code. You can also export each object separately by deciding its size and format and clicking the "Export" button. It is possible to copy the object info by clicking the corresponding button in the same pane.

Lightweight SKETCH file viewer with PNG export option

All things considered, if you're looking for a simple tool to help you view SKETCH files on your PC, Icons8 Lunacy might just be what you're looking for. It comes with a simple interface, lets you export documents and objects to PNG format and also helps you copy object info without considerable efforts.