The GeoServer 1.5.0 beta1 beta release is available for download from the SourceForge Download Page:
Thanks to the Geo-Solutions team for putting all their hard work into this release.
Notice: You will need JAI and ImageIO installed in your JRE in order to run this version of GeoServer. Future versions will contain the jars bundled with the download, and you will not need to manually install them.
Extensions download page
GeoServer License agreement