Having built a few sites using WordPress for a variety of different customers there are a few plugins that I keep using.

• Akismet– Does not stop all spam comments, but certainly cuts down on them.
• CForms – This is an amazing form builder and processor. The bloggers from delcious:days cook good code and good food. Download this from them directly rather than trying to install from within WordPress.
• Events Manager – Both a calendar and an event registration system. For simple community and organizational events this is all you need. Even integrates with Google maps.
• ShareThis – A bit bulking on the amount of javascript code it produces, but a great way to encourage people to spread the word organically.