To be able to publish proprietary formats on the internet using open source techniques, one of the mayor chalanges left is the definition of styling (or export/import of styling from the proprietary system). QGIS not only offers a set of data conversion tools and styling interface, it also contains a mapserver export plugin, with which one can export the styling of a set of layers to mapserver to be published on the internet.
Last year Luca Casagrande launched a SLD export plugin (python). These SLD's can be used to define styling for data published with GeoServer. It's an early version (only supports vector layers with unique values-styling), but it's a great starting point for starting out with SLD and GeoServer. Create styles in QGIS, download the plugin, put it in /qgis/python/plugins and activate it in 'manage plugins' section of QGIS. An 'SLD' button is added to your toolbar, which opens a dialog with the SLD for the styled layers.
For GeoServer there is a great webbased styling tool out there. It's an early version (only firefox 3). The tool is build using GeoExt and uses the GeoServer REST-API (installed as a geoserver plugin) to save the created styles in GeoServer. the tool can not be downloaded with the latest stable geoserver 2.0.2, but it's available with the 2.0.0 version. The styler is also present in the OpenGeo Geo Suite (a suite of opensource GIS applications).