1. Hashtagify.me. This is one of my favorites. Just search for a hashtag and Hashtagify.me will show you its popularity as well as other connected keywords and their relevancy (see the graphic on the left). It also shows the latest trending hashtags, influencers who use the hashtag, and the latest tweets.
2. Topsy. Topsy is a great tool to check on the popularity of a hashtag. It will give you the number of people who have used it in a tweet in increments from a day to a month as well as a sentiment score. There's also a trends chart so you can see if there are certain days when the hashtag seems to be working the best.
3. Twitter Advanced Search. I've always been big on a basic Twitter search at search.twitter.com, but its Advanced Search takes it to the next level. It allows you to zero in on a keyword phrase, a URL, a person, place or date and see the latest trends and tweets.
The next time you're going to post anything that uses a hashtag, check it out on a couple of these sites to see how powerful it is or find an alternative. It could mean the difference between someone reading your post or not.