I love tools that are purely web based.
The benefits are huge - you don't need to install any software, the software is automatically updated by the supplier, it works across any operating system or device or brand as long as it supports modern open web standards. You can carry on working between devices, from different location, and you don't lose your work if your local device breaks.
You can do quite a lot with pure web technologies - Google's office productivity suite is a great example, so is wakari.io's IPython in the cloud.
You can explore the Mandelbrot fractals purely with a web browser too.
I love the following tools I discovered recently: