BANS Tools

https://www.ebaypartnernetwork.com/PublisherHome

Niche Research

http://tools.seobook.com/keyword-tools/seobook/
http://www.wordtracker.com/
https://adwords.google.com/select/KeywordToolExternal
http://www.google.com/trends
http://pulse.ebay.com/
Check your competition
http://www.digitalpoint.com/tools/suggestion/
http://www.keyworddiscovery.com/
eBay Categories

The Five Things That Yahoo! Search Can Do But Not Google | India Inc. - [Stumbles via StumbleUpon]

I found the last one very interesting:

5. linkdomain - An undocumented Yahoo Web search operator.
A quick question - Can you find the number of articles on Wikipedia website that link to CNN.com ? It’s almost impossible to get this data from Google but you can do so quite easily in Yahoo! with the incredibly useful linkdomain operator. The answer is:
linkdomain:cnn.com site:wikipedia.org