Fonts. See a bunch of 'em at once (open source, on a single webpage)

[Darren Dirt]

https://fonts.google.com/

@%&#ing AWESOME interface, fast and flexible.

[Mr. Analog]

There's an app I use called Font Viewer which is pretty good and doesn't require a net connection:
http://fontviewer.net/

It's frustrating looking for a specific font in the sea of fonts that various art programs install (plus the ones I grabbed myself) so this was a natural solution