AZIndex is a plugin that makes it easy to create index pages.

Enhance your blog by creating multiple, three-level alphabetical indexes of your posts based on any combination of post title, author, extract, tags, categories, or custom fields of your choice. Select which posts are included in your indexes using a combination of categories and tags, display them in up to four columns on a single page or over multiple pages.

There is more information at AZIndex for WordPress.

For current plugins at this site: see WordPress Plugins I Use.

Update.
I dropped this plugin on this blog, will do the same on my other blogs.

If you want to keep comment spam to a minimum, install WP-SpamFree.

An extremely powerful anti-spam plugin for WordPress that virtually eliminates comment spam, including trackback and pingback spam. Finally, you can enjoy a spam-free WordPress blog! Includes spam-free contact form feature as well.

This plugin helped me stop a comment spam attack on one of my blogs. Akismet let a lot of the spam through but WP-SpamFree stopped it completely.

I use Secure Form Mailer Plugin For WordPress from Dagon Design on my blogs. There are other plugins like this but I like this one, it gives me all the options I need in an easy way.

This plugin has a wide range of features including: Support for multiple instances, an easy to use dynamic form generation system (any number of fields, in any order), multiple recipients, multiple file attachments, optional auto reply feature, an image verification system, numerous security features (including protection against email header injection), a message template system, multiple languages, and too many other things to list.

This plugin was designed to be easy to use, while still being extremely flexible. If you just want a simple contact form, you can set one up with just a few clicks.

I have switched to another plugin – see WordPress Plugins I Use.

I am testing Sociable on this blog.

Sociable is a plugin which adds social media buttons to your posts, and does so easily, and beautifully.

At present you can choose from 99 different social bookmarking sites, far too many for me but it means you can pick the ones that are relevant for your blog.

The plugin is also available at The Sociable Plugin.

Update
I wanted a better tweet-functionality so I am testing Tweet This plugin for WordPress

For current plugins at this site: see WordPress Plugins I Use.