Hackathon means getting together in an informal setting to play around with technology and learn new skills.

At one hackathon, I wrote isprime. Pass an integer into the URL to see if it's prime.

Algorithm Tutorials from topcoder

CodeCodex – a wiki for finding and improving code

<Coding Ground> is a place you can explore programming languages online

Daily Programmer on reddit has challenges that can help you learn or improve your programming skills

Ideone.com – Online Compiler and IDE

Instructables Contests – some really cool projects and awesome prizes

Linux Commands - a practical reference is a great page that briefly describes quite a few useful Linux commands

Linux Shell Scripting Tutorial (LSST) v2.0 has many helpful pages

Marquette's Student Resources page has brief intros to Unix commands, SSHing, and vi

OverAPI.com has "cheat sheets" for many different programming languages

Project Euler has a long list of programming challenges that you can try to learn or expand your programming and problem solving knowledge.
MATLAB Cody is similar

Rosetta Code has quite a few programs that are in a variety of different languages. Example: 99 Bottles of Beer

SS64 Command line reference

Visualgo and Data Structure Visualization on usfca.edu are two great places to learn about algorithms and data structures

Which Programming Language Should I Learn First? [Infographic] – I have mixed thoughts, but definitely this is a great resource

Paul's homepage