I have launched an addon for Firefox and Chrome browsers at SERPTrends.com recently. This add-on shows SERP changes when searching in search engine.
The idea behind this addon is to know what has changed in the SERP during some period of time. Who gained the positions, who lost them, who is new in the SERP. This can often tell you whether a search engine updated its algo or is it only you who has experienced SERP movements. Or, for instance, you can easily see what has changed in your particular SERPs, find new players and competitors when you return from vacations.
Just wanted to see whether it is useful for you and what new features would you like to see in future. I plan to develop it by adding new features over time.
Any feedback, ideas and suggestions are welcome.