We have previously rounded up useful plug-ins and other notes for voluntary organisations on the popular WordPress web publishing system over on CharityBlog. These articles have been retired, but here’s a possible (and fuller) replacement, The Nonprofit WordPress Guide.
Some of the guide will show its American origins, but most should be relevant more widely.
See a news article on WP Tavern for some background.
Update: also see ‘Extending WordPress for sector websites‘.