For the last couple of years I've been keeping an electronic lab notebook.
For most of that time, it's been a private weblog running on my laptop, based on WordPress with a few small modifications, a custom theme and a bundle of the most useful plugins. It doesn't do anything fancy like version control (at least not yet) or data verification (though there is a nice print stylesheet if you're forced to print out a log every day). It also doesn't attempt to manage all your experimental data - a filesystem, folders and specialised applications should be capable of that by themselves.
What it does do is provide an easy-to-maintain, searchable and browsable archive with an Atom feed, tags-as-categories, a nice admin interface, RSD for autodiscovery of weblog settings (I like to use MarsEdit for posting), a live calendar and text formatting using Textile. Now I've decided to release this electronic notebook as a package for anyone to use, called NotePress: it's licensed under the same terms as Wordpress (GPL) and downloadable using Subversion (for instant updates to the latest version using svn update) or as a compressed file archive.
Here's the main page for download and discussion of NotePress, with installation instructions (please feel free to add your own detailed instructions if you run Windows or Linux).