One of the servers that I help operate used to be host to Robot Wisdom. Net people of a certain generation probably know what that is. (If you don't, you need to go check out Jorn Barger's Wikipedia entry right now.) By many accounts, it was the world's first weblog, and it was started before WordPress or LiveJournal. Hell, before PHP. It didn't use a database at all. It was just a big mess of text and HTML files and a few images. The complete site tarred and gzipped was a few megs, I think, even though there were archives going back to late '97.
When I started thinking about resurrecting the blog, I found myself getting more and more interested in having a model closer to this. Dynamic web applications are really necessary to the web we all know and love, but blogging, or at least running a small personal blog, is really only one step removed from writing a letter or publishing a newsletter. You write a post, you modify some links in other parts of the application, and you publish. Using a blogging engine to achieve this seems a bit like overkill.
Basically, I was looking to make the process of writing much like any other development project: maintain my normal edit/debug/commit/push workflow with my preferred set of tools. I was looking for something small to automate all the monkey work specific to the writing and publishing process. Jekyll was just what I was looking for.
I like that it stays out of your way. You design your templates the way you want them, write your content in the markup language of your choice, and run a job from the command line to regenerate the site when you're done. Since you're working with flat files, you can check the whole project into your favorite source control system, and manage it like you manage any other project you work on. GitHub has provided an implementation of this already in their Pages feature. It's worth checking out if you're looking for a simple, turnkey solution or if you want to use it as a model for your own custom workflow.
I also didn't have to wade through all the heavyweight features that come standard with every modern CMS in order to put the site in order. I didn't need to read a reference or learn a bunch of PHP library calls in order to put together a theme. I just defined the templates using plain old HTML and CSS. It's a way of working that's much more easy and natural for web-literate people.
The only thing I thought I might miss by not having a dynamic, database-backed site was the ability to handle comments. (Maybe. Hypothetically.) But Disqus made it easy enough to move that functionality off my server. If I decide that there's a feature I can't live without in the future, I'm always free to hack it together myself using JavaScript and maybe a simple web service. That's a little more satisfying to me, and it acts as a natural check against the plugin-itis that afflicts so many other blogs.
The gem installs with support for migrating from a few of the more popular blogging engines. I used the WordPress converter class to extract directly from my old blog's database. The whole procedure was quick and simple and dumped each post into a separate Markdown file. I did have a few issues with some of the files where I had diacritics in various languages. Other than that though, it worked like a charm.
Jekyll is hosted on GitHub. At the moment, it's got a lot of open issues, so if you're using it or think you might want to, please consider donating a little bit of your time to fork and contribute.