The Annotation Tool extension solves two annoying problems with the built-in Notes feature in OpenOffice.org: you can't attach a note to a highlighted text segment, and you can't delete all the notes in the current document in one go. With the Annotation Tool you can do both.
OpenOffice.org doesn't natively support the SVG graphics format, which can be a bit of an inconvenience, especially considering how many good vector-based images are available in the Open Clip Art Library. As the name suggests, the SVG Import filter adds support for the SVG format, so you can easily open vector-based graphics directly in OpenOffice.org. According to the author, the filter produces nearly perfect results for simple SVG files, though there are still some issues with color and transparency gradients. When downloading the SVG Import Filter, you might want to take a look at the svg2office batch-processing utility, which can convert multiple SVG files into the OpenOffice.org native graphics format (odg).
OxygenOffice Professional adds a lot of extras to the stock version of OpenOffice.org. While you can download OxygenOffice as a complete office suite, all of its components (clipart and templates) are available as separate extensions. They can come in handy if you want to add only selected templates to your OpenOffice.org installation.
The Tabbed Windows extension does pretty much the same as the Tabbed windows feature in Firefox, and it can be a great time-saver when you have to manage multiple OpenOffice.org documents at the same time. The current version of the extension is still at a very early stage of development, and as such it can be a bit unstable and has a few limitations. For example, all the tabs are named "my windowX" and there is no way to rename them. You can't reorganize tabs either.
Strictly speaking, OOo.HG is not an extension but a huge collection of maps and mapping elements stored as OpenOffice.org Gallery files. Using OOo.HG, you can turn OpenOffice.org into a powerful map design tool. The core of OOoHG consists of three libraries. CART'OOo is designed to quickly create maps and geographical drawings. It consists of 207 vector-based geographical and historical maps divided into 13 themes and 153 "geographical objects" that can be used to create maps, legends, drawings, etc. The CHRON'OOo library is designed specifically for creating chronological timelines using the supplied objects. ATLAS'OOo allows you to create bitmap maps using 226 geographical and 187 historical maps.
For more extensions, visit the Extensions Repository page of the OpenOffice.org wiki.
Dmitri Popov is a freelance writer whose articles have appeared in Russian, British, US, German, and Danish computer magazines.