Plugins can be addictive. As with themes I recommend you keep these to a minimal pattern. If there is anything you can do on your own without plugins: ie; tags in the header, favicons, etc, I recommend you do it manually. Plugins can be a doorway to hackers. At first you will be tempted to try every plugin and that is fine, but have a long term goal of reducing them in number to 15 or so.
Before I list my favorite plugins, I wanted to give you a tip on a weight loss pill. You should weigh every decision based on its merits. This website can help you. I hope it does if you are in the market for it.
Now for the listing: My favorite plugins vary but for the moment I’d say my top ten are:
WP-PostViews, @Reply w comment preview, AddThis Social Bookmarking Widget (drop-down), All in One Adsense and YPN, HeFo, In Series, Nofollow Links in Posts, Popularity Contest, Subscribe To Comments, and Twitter Tools
Have fun exploring plugins but remember, there is a reason WordPress doesn’t include them in the core. Use them at your own risk.