I always prefer one of the best & free seo tool which is google adword keyword tool. This tool is very helpful for researching keywords for seo. There are some other things you can do with this tool to improve your keyword research and discover additional relevant keywords.
It depends on what I'm doing...
To spin articles, I use the Best Article Spinner
To create social media bookmarks, I use bookmarks I use Bookmarking Demon.
For backlink analysis I use SEO Spyglass.
I also employ Rank Tracker to track keywords on big projects.
And, last but not least, Google Analytics to analyze traffic :)
iBusinessPromoter or IBP seems like a good choice still the price is overrated compared to what the actual benefit of using the tools.
One should be able to get.
There are also Seo Elite, Web Ceo, Submit Suite, Fast Directory Submitter that are quite good.