A collection of several binary-reversing bomb defusing challenges.
pwnable.kr is a non-commercial wargame site which provides various hacking challenges regarding system exploitation.
A series of 10 challenges, each of which requires supplying the correct password to a binary file.
Microcorruption is an online embedded-security CTF. Given a debugger and a device, solving each level consists of finding an input which will unlock the device.
Providing a hands-on approach to learning web security, PentesterLabs has both a bootcamp curriculum and several exercises available for free (in addition to paid content). Working through their bootcamp and doing the exercises is a great way to start learning web security!