WP Plugins YOU should know about

[webarchitect]

Well I hope this thread will give us some insights on the best plugins to use (for tags, SEO, images, etc.) for WP

[hermione]

There is another thread here about WP plug-ins: Favorite Wordpress Plugins

[wwstewart]

Here's a couple that I love.

FireFTP - In-browser FTP client
Sage - RSS reader
StumbleUpon - stumbles things.
TinyURL - creates short URL's from your browser
Greasemonkey - I won't explain...but you need this.

[MasterYourCard]

I highly recommend the 'Related Posts' WP plugin. Really good for keeping readers on your site(s).

It can be a little tricky to install, but it's well worth it: http://wasabi.pbwiki.com/Related%20Entries

[amyschiff]

I've not used word press much, but a plugin I came across that might be useful is Clean Notifications

[hermione]

I've been using WP for a couple of months now and I love it because the plug-ins are so easy to install. (Unlike Zen Cart and OsCommerce sites where installing modules and add-ons are real pain in the butt complicated processes).

But while WP plug-ins are great, I have found that the fewer you can do without, the better. The more plug-ins you have installed, the longer it takes for your site to load. If you do a Ctrl U on one of your WP pages, you can how many of the plug-ins that page loads even if that plug-in isn't used on that page.

So, if there is a plug-in you don't really need, I'd suggest removing it if you want your site running as fast a possible.

[webarchitect]

I've tried using the RSS plugin, and that all-in-one SEO pack...seems like good plugins to have.

[amyschiff]

A new one recently released is the commenter info plugin which gives you and your readers info about your commenters' posting habits in the form of a tooltip type thing

[Jason White]

I recently installed a plug-in called "Better Comments Manager" and have been pleasantly surprised by my improved ability to respond to comments. It's nice not to have to visit each post individually to respond, or do it from the traditiona WP dashboard.

[sxyshandy]

I'd agree with @Hermione to keep the plugins at a minimum, especially because when there are too many "hacks" they can conflict with each other or with later Wordpress versions. I should know; we have to go through this with some of our Wordpress clients.
Here are some 'essentials' I recommend:
Google Sitemaps - generate your www.yourdomain.com/sitemap.xml and then submit to your Google Webmaster Tools account (yes, you should have one)
Feedburner Feedsmith - routes feeds to your feedburner feed url so that www.yourdomain.com/feed goes to http://feeds.feedburner.com/yourblog (Yes, you should have a Feedburner account)
Akismet Spam - of course. Requires an API key from your wordpress.com account (Easy to register for one)
Share This or Add This to add social bookmarking widgets to each post. Or get your feedflare HTML code from your feedburner account
Subscribe to Comments - really useful for commenters to know if you've responded to them and so they can follow the conversation