I find the following plugins useful as a set of default plugins for a WordPress installation:
- ultimate-google-analytics – requires an analytics account (free) and really is the ultimate SEO plugin;
- google-sitemap-generator – generates a Google Site map (useful with a Google Webmaster Account);
- platinum-seo-pack – Manages Title tags and meta tags along with loads of other SEO tools;
- revision-control – controls (limits) the number f revisions you accumulate;
- WPTouch – Provides an iPhone interface to your blog.
- Akismet – Comes with WordPress – activate it – it works and really stops a lot of comment spam.
I load all of these by default with WordPress installations. The SEO ones really enable you to manage and monitor the visibility of your site and to track traffic.