1) search.twitter.com is more of an all-purpose Twitter search site. If you're looking for any keywords related to hashtags, be sure to place one in front of your search term (like #keyword). If you select "advanced search" you can make your search even more granular.
2) Twellow is a Twitter yellow pages directory. It's interesting in that if you search for a keyword, you'll get the profiles of users with that keyword in their profile based on who has the most tweets.
3) Tweetstats and Tweetstats Trends are a good way to look at not only what your personal stats are, but also the latest trends on Twitter.
4) Trendistic is another Twitter tool that will show you the hottest trends of the day, week or month, as well as how any particular keywords are trending.
I hate to say it, but that's about it. Search.twitter.com is the closest thing to hashtags.org, but you have to remember to insert the hashtag before your keyword. Remember that a quick look at both the searches and trends before you select a keyword can make a difference in your Twitter traffic and ultimate attraction to new followers.
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