I hope Holla Forums will be interested in these kinds of threads, I can think of many improvements.
CONTEST SITES:
hackerrank.com
hackerearth.com
codechef.com
spoj.com
uva.onlinejudge.org + uHunt: uhunt.felix-halim.net
RESOURCES
Competitive Programming 3
drive.google.com
Competitive Programmers Handbook
cses.fi
github.com
topcoder.com
Solve problems starting from the most solved ones:
www.spoj.com/problems/classical/sort=6
or
codeforces.com
It is fun, challenging and can greatly improve your algorithmic, optimization and general language skills.
There are numerous contests organized, with many having prizes.
Knowing Competitive Programming can greatly help you in job interviews.
Share resources, must-solve and interesting problems, language-specific tip and tricks, or whatever comes to your mind!
Tommorow is Google Code Jam round 1B, don't forget to participate if qualified.