The only software I use is the big nerve between my ears :-)

I sometimes use this to see what a spider sees:
http://www.1-hit.com/all-in-one/tool...ine-viewer.htm (This is just one of many available - search Google for search engine simulator).

I usually validate the HTML:
http://validator.w3.org/

I think you are really asking about software such as Web Position Gold which I never use and is not free. There have been a few threads here about it.
My main reason for not using these types of software is:
1) They really focus your attention on the on-page factors, which now are really not as important in the current ranking algorithm in Google (off-page stuff is more important)
2) They have very precise formulas for placement and density of keywords ... Google have just gone through 3 major algo changes ... ie the rules have changed.
3) It can make a page look 'unnatural'
4) Google specifically recommened against using WPG to submit (I know that what not your question)

CBP