Hashtagify.me is a neat site that tracks hashtags in Twitter. For example, here is what they have on #GameGate. They show the other hashtags that your hashtag connects to (like #NotYourShield) and you can get a trend line.
The trend makes it look like #GamerGate is going down, but I don’t trust their projection.
All of this is free. They also have a Pro account, but I haven’t tried that.
Thanks to Brett for this.