Here are a few plugins I use on most wordpress installs:
- Akismet - Due to wordpress' popuplarity, many spammers will visit your website. This antispam plugin comes standard and is free for personal use. You will have to retrieve a key first using a wordpress account first though
- Posts 2 posts - allows many to many relationships between posts with optional link info
- Fast Secure Contact form - quick contact forms sent via email. A nice companion plugin is the to use is the database storage plugin to store entries to a database.
- Custom content type manager - easily add multiple content types. The nice part about it is multiple content types get their entry in the dashboard and it provides multiple custom field types that are repeatable.
- Google analytics - keep track of visitors
- Google adsense - put ads in your websites
- W3 Total cache - when performance issues arise, this helps
- WP Mail SMTP - Many hosts either provide no mail server or their ip is blocked from sending mail. This plugin allows to send email via an alternate provider such as gmail
- Wordpress SEO - this plugin, among others, rewrites headers to be more search engine friendly. An alternative is All in one SEO pack which has been around much longer but has been losing in popularity.
- next gen gallery - for all times you need to add picture galleries, this is a good option. Many extensions to add flexibility exist such as voting.