My latest project is a bookmarking app that uses HTML5 localStorage. I've purchased a more memorable domain name, which I'll reveal after DNS propagation.
The app now lets you winnow down results by tag (in the left column). Also, after you click on the bookmarklet to add a tag, a prompt pops up to let you add tags.
I've been using a self-hosted PHP app called Semantic Scuttle after having used Pinboard for years. I have no complaints about Pinboard, except that my data is stored on someone else's server. With Semantic Scuttle, the data is stored on my server, but it's still on a server. Why not just store my data in the browser's memory? The downside is that the data is not available across all my devices, but if a bookmark is only really relevant for the device on which it is saved, then it's not a big deal. Also, I can pretty easily export my data in JSON.
Dave Winer has written a number of apps running on Node.js, which rely on Twitter for identity and Amazon S3 for storage. I am interested in using a solution like this to store data in the cloud "on the cheap," but I currently lack the understanding and skills to adapt nodeStorage for my own purposes.
I'm also doing this to develop my object-oriented programming chops. I've already written apps to store favorite bands and social media profiles, so it gets easier and easier for me to write apps like this. And once I've solved a programming problem, I can re-use the solution in a newer app.