Do you mean keyword research?? There are a lot, wordtracker, wordze, keyword discovery, keyword spy ..... If you do not want paid services, you can try to combined the results from a few free tools. wordtracker and keyword discovery offer some free tools. And I think you can search here in DP there are a lot of free tools mentioned before.
inventory.overture.com/d/searchinventory/suggestion/ This is the best tool for finding keywords and its from yahoo and totally free
the OP is asking for keyword difficulty tool so besides the seo-chat k/w difficulty tool, i'd suggest SEOMoz k/w difficulty tool ( subscription reqd) otherwise, you can always use the good old intitle inanchor command in Google to check out how many sites are seriously competing for the targetted k/w hope this helps
I find the very best tool for this is Google itself by using the allinanchor, allintitle etc operators.
Nice tool when the dam thing works right .. 1/3 is the best I could pull off my keywords questions. Yikes..