The theme that I am using for this blog, Andreas09, allows me to add widgets to the sidebars. And most of these are customisable to varying extents. While I have selected a few for mention here, there are others like Tag Clouds (very Web 2.0 and, I think, very inelegant) available too.
The most customisable of all is the Text widget; you can add practically any (X)HTML to it, which is how added my online photo gallery teaser. Don’t try adding any scripting, though, or it will be removed for security reasons. Even JavaScript suffers this inglorious fate, and I wouldn’t be surprised if it was the same for PHP.
Next up in usefulness, at least from the content point of view, are RSS feeds (just look for the headings with the orange logos beside them). The ability to show shared items from your Google Reader account is a nice piece of convergence. Speaking of convergence, I also added the feed from my hillwalking blog too. Taking things further again, I have added ones for InternetNews, A List Apart and The Blog Herald and I wonder if RSS feeds will not replace email newsletters now that we have tools like Google Reader.
Moving to the navigation side of things, the Categories widget can be collapsed to a dropdown menu, like I have for the Archives one. I prefer things to be the way that I have them because I want people to see what’s here. The Calendar widget makes up for visitors not spotting what the dropdown represents; that’s why the Archives widget can be a dropdown menu rather than a list.