A short list of plugins I like and use

Askimet – of course – say no more a must for every website

bbPress – Forum software that works with your theme – so looks fully integrated

bbPress WP Tweaks – adds an additional forum specific sidebar for use on forum pages – great for putting logon, recent topics etc.

bbPress signature – adds a signature to posts

Restrict Content – lets you set pages and posts for certain levels. I use it for membership only pages, letting the world see most of the websire, but restricting parts to members who need to log on.

Marketpress lite – easy to use, has some tiny niggles, but it free !

CformsII – Not the easiest forms package, and has some niggles – like you need to save changes regularly as you work with a form or sometimes it loses stuff you’re setting up – but works well.

Recent Posts Category widget – displays the most recent posts in a particular category – I use it on competition pages to show the latest results in the sidebar.

Widget logic – Control widgets with WP’s conditional tags eg  is_home