I've been to ICPC world finals twice. Once in 2010 in Harbin, China and again in 2011 in Orlando, Florida. In Harbin we ended up ~35th place (technically they ranked us in a massive tie for 15th place I think) and in Orlando my team, including Jonathan Plotzke, Qifeng Chen, and myself, managed to rank 2nd and grab a gold medal. This is the second gold medal that the USA has won since 2004 (when the history on Wikipedia stopped).
TopCoderCheck my TopCoder profile.
I've been to the TopCoder Open in 2007 and the inaugural Top Coder High School tournament in 2007.
CodeforcesCheck my Codeforces profile.
SPOJCheck my SPOJ profile.
USACOI help setup monthly contests for USACO. This includes writing problem statments, creating solutions, creating test data, and writing analysis.
IOII've only been involved with the IOI as a non-contestant. In 2011 I was a beta tester for the problem set and in 2012 I served as a coach at the USAICO training camp that selects the team for the IOI. One of the contestants from the USA, Johnny Ho, received the only full marks that year.