Orb’s Brush Pack – This brush pack contains the famous “Orb Cracks” brush, which I personally prefer using over the dmstandard brush that comes with ZBrush.
zbrushguides.com’s ZBrush Brushes – a beautifully-presented collection of both free and commercial brushes.
Michael Dunnam’s Brushes – A forum thread on ZBrushCentral showcasing Michael Dunnam’s custom brushes, almost 200 of which are available for free (check out his Gumroad to get straight to the download page).
Badking.com’s Brushes – an impressive collection of free brushes.
Jonas Ronnegard’s Gumroad – free stylized rock brushes and commercial brushes.
ArtStation Marketplace: Brushes & Alpha Packs – a platform for artists to sell their products, alphas included.
Want your brushes added to this list for free? Contact me.