Keyword Research Tools

• Keyword Tool by Google AdWords
• Google Insights for Search
• SEO Book Keyword Suggestion Tool - Free but requires registration
• Ranks.NL Keyword Density Analyzer - Search engine metrics for specific page
• David Naylor Keyword Density - Metrics about content

Webmaster Tools

• Google Webmaster Tools
• Check Page Speed Online
• SEO Book Webmaster Tools - Free, but require registration.
• SEOMoz Internet Marketing Webmaster Tools - Variety of many usefull SEO Tools

Link Popularity and Analysis Tools

• Open Site Explorer from SEOMoz - Link popularity checker & backlink analysis tool

Twitter Tools

• Hoot Suite - One of the best Social Media management applications. Allows you to post to Facebook pages and Twitter Accounts from a single admin dashboard, schedule posts at later times and tons of other useful features. Upgrading to Pro allows unlimited social accounts plus addition of multiple administrators who can manage posts.
• Co Tweet - Set up future tweets and post them to multiple account from one location
• Manage Flitter - Very useful tool for cleaning up your user followers. Multiple user deletion also

Find Followers

• Twitter > We Follow - Reasearch and add users found on Twitter. Search by area, fields or subjects.
• Twitter > FollowerWonk - Another site offering to find and follow Twitter users by Specific Industry.
• Twitter > Listorious - Another site offering to find and follow Twitter users by Specific Industry.
• Google+ > Find People on Google+ - Google+ is becoming a formidable player. Find people you can follow and add to your circles.