Microsoft has released a trail version of Windows Small Business Server 2008 Premium

This software is for evaluation and testing purposes. Evaluating Windows Small Business Server 2008 does not require that you activate the product or enter a product key. If you need more time to evaluate the software, you can reset (or rearm) the 60-day evaluation period up to three times, extending the original 60-day evaluation period by up to 180 days, for a total possible evaluation time of 240 days. After the evaluation period ends, you need to uninstall the software or upgrade to a fully licensed version of Windows Small Business Server 2008.

Windows Small Business Server 2008 Premium Trial