Adobe CC PhotoshopImage via Fstoppers

Photoshop alternatives

Acorn (OS X, $49.99)
Pixelmator (OS X, £20.99)
GIMP (OS X, Windows, Linux, free)

Illustrator alternatives

Sketch (OS X, £54.99)
Inkscape (OS X, Windows, Linux, free)
iDraw (OS X, £17.49)

InDesign alternatives

Quark (OS X, Windows, £799)
Scribus (OS X, Windows, Linux, free)
Lucidpress (browser-based, free while in beta, subscription to follow)

These and a few others mentioned elsewhere:
Anthony Colangelo of Happy Cog reviews Sketch, on A List Apart
Jon Hicks uses Acorn and Sketch
Khoi Vinh on Sketch
Alternative apps to everything in Adobe Creative Cloud, on Mac.Appstorm
15+ alternatives to Adobe Creative Cloud, on Web Designer Depot

If you've used any alternatives it'd be great to know what you think.