I've been trying out a lot of web applications lately. Web applications look like normal web sites, but they provide functionality equivalent to desktop software. For instance, I'm writing this post using Writely, a web-based word processor. It features spell checking, formatting tools and the ability to save your work in Microsoft Word format. It runs totally from my browser without installing anything, and anything I write is stored on their server as well. Some other cool features include auto-saving, direct posting to a blog and allowing multiple users to work on the same document together.
Another application I've used recently is Ta-da List. The name may be a bit goofy, but it's fast, free and easy to use. Basically it keeps track of your to-do lists, allowing you to share them with others as well. It may sound simple, but simplicity is what makes it good.
Of course there's Google Maps too, which surely doesn't need any introduction. I've not used it other than as a novelty item, but it's extremely cool nonetheless.