Clicktracks For stats which has some great tools for tracking and analysing vistors, but its the high priced spread for sure.
Keyword suggestion tool for a free and easy way to research keywords on Wordtracker and on Overture at the same time.
Wordtracker for brainstorming keywords. You can get a one day subscription for about $7.00 and that is enough to reasearch one sites keywords.
WebPosition gold for comparing the details of your pages optimization with up to five other top ranking pages (and though many will tell you its a no-no it has a great ranking tool also)
But the best resource of all is located immediately beneath your hair - fed with facts and used properly it is the best of all.