Gadgets and other scripts: As a community member I maintain/write many gadgets and site-wide scripts on numerous wikis. I often fix bugs, update versions, clean up and refresh old gadgets etc..
Toolserver: Maintenance and creation of tools on the Toolserver. On User:Krinkle/Tools is a fairly complete overview of the different tools I created. I'm also creator of the Intuition project (for more info, check the Intuition project pages at translatewiki.net).
MediaWiki.org: Writing technical documentation on MediaWiki.org.
Countervandalism Network: Since early 2010 I'm a member of the Countervandalism Network (senior staff). An unofficial semi-formal organization independent from the Wikimedia Foundation destined to join forces against vandalism by offering hosting for and building/maintaining tools. Sub-initiatives: Small Wiki Monitoring Team and CVNBot. See also CVN Server admin log.
Wikimedia wikis: Although my account is unified on all 800+ wikis, I'm mostly active on Wikimedia Commons and the Dutch Wikipedia, both on which I'm an administrator doing maintenance tasks (patrolling edits and uploads, counter-vandalism, copyright violations, etc.). See also my global account overview.
Disclaimer: Unless explicitly otherwise stated, any contribution I make to Wikimedia wikis is an act from my volunteer role as member of the community; not a legal or official action from Wikimedia Foundation staff.