What free wordpress plug-ins do you use most?

I use a dozen or so! Here are my absolute fav's!

Advertisement Management - Allows you to insert ads into your posts etc.
Akismet - Helps control comment spam
All in One Adsense and YPN - Adsense and Yahoo Ad integration
All in One SEO Pack - Helps optimize your site and post content
Bad Behavior - Works with Akismet on controlling spam comments
FeedBurner FeedSmith - For use with google's RSS Feedburner program
Google XML Sitemaps - Every site needs a sitemap! This is the plugin for building one for your WP blog the right way.
Permalink Redirect - Makes your post titles search engine friendly.
SexyBookmarks - The top social bookmarking buttons integrated into your posts.
Show Akismet Count as Plain Text - Shows how many spammers have been blocked and displays it on your wp blog.
Simple Tags - Based upon your previous posts & tags, simple tags displays "quick tags" basically so you don't have to type in the entire tag.
WP-UserOnline - How many users online right now? Counts most ever online at one time and tracks I.P's and referral URL's. Great for tracking your most popular posts by seeing the pages your visitors are actually on right now.
WP OnlyWire Auto Poster - Works with OnlyWire service for auto-notifying social bookmarking sites and others like StumbleUpon and Mixx for instance, about a new post on your blog. Once set up, it works on auto-pilot.

I only use All in One SEO Pack and Google XML Sitemaps for my wordpress blogs. As i don't like to install a lot of plugins on my blog because i know i will not use most of them.

I try to keep the number of plugins to an absolute minimum as they are a security threat. I never go out and look for new cool plugins, I only use them when I have a true need for them.

Does anyone feel that a large number of plug-ins slows down your website's load time?

the database back up plug in
very important!

Good point, does it save something to your desktop?

I use several free and paid max blog press plugins. Free ones include: Max Favicon, Max Ping optimizer, Max banner ads, Max Optin Form adder.

The free ones can be found below.

Max Blog Press Free
Plugins

All In One SEO pack, Askimet, keywordLuv and Google sitemap generator are my favorites.

I normally use DoFollow plugin to reward useful comments.

I keep plugins at a bare minimum. So many of them do things that you can do within wordpress's settings file that it's not worth it.

I use All-in-one Seo, WP-spamfree (easier to set up than Akismet, though not as effective. I don't really moderate comments anyways), DB Backup, XML sitemaps, and Sociable.