Similar to Coq

JACK Audio Connection Kit

JEdit

Jekyll (software)

Jetty (web server)

JFreeChart

Jigdo

Jitsi

JRuby

JSDoc

JuK

Jython

K-Meleon

K3b

Kaffe

KAlarm

Kali Linux

Kate (text editor)

KaTeX

Kawa (Scheme implementation)