[turboponyy]

It's quite nice for the following reasons:

- Image previews (for file management with dired/dirvish)

- PDF viewing

- In-buffer images (e.g. profile pictures in git log with Magit)

- Browsing simple HTML pages (e.g. API docs)

There's probably more I've yet to discover.

[anthk]

M-X calc and/or imaxima with embedded Gnuplot generated plots.