Here is a collection of links to sites where you can practice and hone your 1337 hacking skills
An up-to-date list of upcoming CTFs.
Our go to practice CTF for beginners to experienced flag capturers.
Less hand holding than Pico, but still a nice mix of beginner and expert problems
A CTF based around emebedded systems (hardware)
Web challenges only! The beginner problems are well documented on other sites, but they get hard quick.
A much more comprehensive list if you are looking for something else.