Microsoft Research has released an add-on / Extension for Internet Explorer which helps you find what changed on any website when you visit it again. It maintains a time stamp so you can compare according with respect to the real time. All the changed sections / texts are highlighted in yellow color and it includes any changes in the images also i.e. in case file name of an image is changed.
This add-on is extremely useful for people who want to track a page where a deal opens up on a particular day or a major update is due. Also you can use it on forums to find what kind of update was made both in terms of edit and new answers.
This add-on comes with few basic settings also which lets you control on things like caching, image caching, ignore SSL and maintain a list of sites which can be ignored all the time.
To compare changes side by side you will need to change caching setting to Image i.e. store snapshot as Image which makes it easy for comparison.
For best use of this add-on you should stay on a website for couple of minute so the trace completes and then come back later to find the changes if any. Download DIff IE