Things to help you understand which SEO/keyword markets to attack?
KGen

I nearly flipped my lid when I first used this plugin for Firefox. It?s a sidebar that ? at the push of a button ? scans whatever page you?re on and gives you a read on its keyword volumes and keyword density. It also provides this information as a natty, visual tag cloud for those with autistic tendencies like myself. You should use it for snooping on competitors. If they?re good at their game, you?ll learn heaps about why. I promise it?ll blow you away.
Google Adwords Suggest

Just type in whatever keywords and/or phrases you?re investigating, hit a button and this tool will tell you how many people have used the same verbage to search Google in an average month, and also how many competitors are out there bidding on the same terms as part of their PPC ad campaigns. When it comes to content planning, I?d be lost without this one. It tells me whether or not I have an audience for my keywords, and whether there?s any competition out there. As such, it helps me know what content to write, how to do it and how to promote it.
Wordtracker

Like Google Suggest, but provides (independent) data on keywords from a wider variety of search engines. Basically, it keeps a three month store of activity on Google, MSN and Yahoo, and tells you how many times a day your keywords have been used by the world at large. In addition, it gives you a superb competitive index that tells you how many other interweb pages are optimised for your terms. This is gold dust as it enables you to get a true measure of whether a Content Optimisation strategy is worth pursuing or not. (What looks interesting from a search volume perspective may often turn out to be a monster when it comes to competition!) Costs a couple of hundred bucks a year.
Competitive Online PR & Content Optimisation Tools

Things to help you understand what black magic your competitors are practising?
Keyword Spy

A very smart widget that shows you which other companies and/or sites are using your keywords for their PPC campaigns. Really useful for getting a quick view on who you may be up against on your new Adwords campaign (and for realising which keyword markets to avoid!).
Keyword Page Comparison Tool

From Aaron Wall?s wonderful SEObook.com. This rinky dink tool enables you to grab a quick read on the technical composition of a web page by scraping its title, meta description, meta keywords, page copy, and top keyword phrases and presenting it all back to you in one place. In addition, it allows you to do this stuff on a comparative basis, so that you can do a competitive bake-off between sites. And, even better, it lets you download this data as an Excel file, so you can get jiggy with it when you?re offline.
Keyword Density Tool

Again, via SEObook, this is a variant on the Keyword Page comparison tool, but gives you a bit more flexibility to include and exclude certain paratmeters. Great to use to get a rapid view on how well your competitors are thinking about keywords and SEO. (I use it to see if competitor pages spit out a range of meaningful words, or just a bunch of crap. If it?s the former, then I know I need to really get with the program; if it?s the latter, then I know I can make serious a difference to relative performance with some simple Online PR and Content Optimisation work.)
Online PR & Content Optimisation Dreaming Tools

Things to help you get creative when you?re in a tight spot?
Google Trends

Very neat service which plots you a volume graph over time for ?big fish? keywords (phrases that are being used by lots of searchers). Also tells you which geographies this traffic is drawn from, and gives you pointers as to the key news pieces that may have driven keyword interest. Won?t give you any data on smaller keyword markets, but certainly provides you with a firm handle on the keyword zeitgeist via its read of daily trends.
Thesaurus.com

Because we all need a little help with our dreamscaping.
Dictionary.com

Because we also need to spell words correctly, innit?
SEO, Online PR and Content Optimisation Analysis Tools

Things to help you understand your performance and your competitors (and keep you amused deep into the night when you really should have gone to bed a long time ago)?
SEO Quake

The daddy of SEO analysis tools. A plugin for Firefox that sits as an additional toolbar at the top of your browser window. When you?re on a page, it?ll tell you (immediately) key things like Google PageRank, page index volume, volume of inbound links, volume of external links, and other essential data. DON?T LEAVE HOME WITHOUT IT!
Xinu

This is an awesome little service that gives you an instant read on a site?s SEO performance across a wide range of metrics. At the press of a button you?ll see key indicators like social media footprint (how often a site?s been bookmarked), volume of backlinks (and their source), and number of pages indexed in key search engines.
Google Analytics

If you?re not using Google Analytics to understand your traffic sources, your top performing content and your keyword performance, then you need your head examined. Also gives good readings on important stuff like bounce rates, time on page, average page views and the like.
Opentracker

Much like Google Analytics, but has a cool feature that shows you which companies are browsing your site IN REAL TIME! (So that you can pick up the phone and harass them.) It does this by checking a visitor?s IP address against the Whois database. Costs peanuts a year and worth subscribing for this feature alone.
Google Webmaster

Lots of Jedi insider sprockets here to help you understand how often your sites are being indexed and which pages are being accessed. Also shows you what Google actually sees when it gets to your site. Invaluable!
Blog Content Research Tools

Things to keep you in the know and amongst the buzz and gossip?
Blogpulse

Kind of like a Google for blogs. Also free. Just type in a search term and it?ll give you back a ream of related (recent) blog posts. You can also do some neat ?trending? vs other keywords to get an idea of what?s hot and what?s not (an essential skill for any Blogsphere Gal-About-Town).
Twitter Search

A Google for Twitter. See who?s talking about you and your keywords. Then go and hide if it?s something you really didn?t want to hear. A fab time-sink for brands ? as you can now see what the world?s saying about you in near-real-interweb-time. (Or at least listen to the Twitter crowd to check who?s bitching about you.)
Run of the Mill Online PR Tools

Things to help you spread the word and generate some backlinks at very little cost?

I?ll make it a list. They basically do the same thing: distribute your press releases around the interweb at next to no cost. Very handy! (They also tend to use clunky spliced verbs in their brand names. Go CheckWatch them out!)

* PitchEngine
* FastPitch
* ClickPress
* Sane PR
* PR Log

Social Media Tools and Other Online PR Goodies

I really have to thank my friends at Nixon McInnes for prompting me to write this post, as they?ve also been sharing some great Social Media tool-age via their latest (and very excellent) social media agency newsletter. So go check them out. They introduced me to the MIND BOGGLING MonitorThis RSS tool. I shall forever love them.