Brian Bowman has a very useful post on the Reply.com blog on Tools for SEO and SEM at:
http://blog.reply.com/?p=265

Here's the post;

Essential SEM & SEO Tools

Posted by Brian Bowman, CMO, Reply.com

We always get asked our opinion on cool / essential SEM & SEO tools. Well, here is our list.

Google website optimizer
What it does: Free multivariate and A/B testing tool, helps online marketers increase visitor conversion rates
Cost– Free
services.google.com/websiteoptimizer/

Google Analytics
What it does – Free analytics program
Cost – Free
www.google.com/analytics/