SQL Server 2008 Spatial, PostgreSQL/PostGIS 1.3

General Overview
FeatureSQL Server 2008 (RC0)MySQL 5.1/6PostgreSQL 8.3/PostGIS 1.3/1.4
OS
Windows XP, Windows Vista, Windows 2003, Windows 2008
Windows XP, Windows Vista, (haven't tested on 2008), Linux, Unix, Mac
Windows 2000+ (including Vista and 2003, haven't tested on 2008), Linux, Unix, Mac
Licensing
Commercial - Closed Source, Various levels of features based on version, Express version has full spatial support but limitation on database size and only use one processor.
Commercial Open Source (COSS), some parts GPL. Here is an interesting blog entry on the subject MySQL free software but not Open Source. The comments are actually much more informative than the article itself.
FLOSS (PostgreSQL is BSD, PostGIS is GPL Open Source - you can use for commercial apps but if you make changes to the core libraries of PostGIS, you need to give that back to the community)
Free GIS Data Loaders
shp dataloader for SQL Server 2008 developed by Morten Nielsen (doesn't yet work with RC0)
OGR2OGR, shp2mysql.pl script
included shp2pgsql, OGR2OGR, QuantumGIS SPIT, SHP loader for PostGIS also developed by Morten using SharpMap.NET various others
Commercial GIS Data Loaders
Manifold, Safe FME Objects, ESRI ArcGIS 9.3 (in a later service pack)
Safe FME Objects
Manifold, FME Objects, ESRI ArcGIS 9.3
Application drivers available specifically for spatial component
? Not yet - SharpMap.NET eventually and probably built into new ADO.NET 3.5+
GDAL C++, SharpMap via OGR, AutoCAD FDO
SharpMap.Net, JDBC postgis.jar included with postgis, JTS etc. tons for Java, GDAL C++, AutoCad FDO beta support
Free Object/Relational Mapping
NHibernateSpatial (this is a .NET object relational spatial mapper) - beta support
Hibernate Spatial - this is a java object relational mapper
NHibernateSpatial and HibernateSpatial
Free Desktop Viewers and Editors
Will be built into SQL Manager, but not available in RC0 and only useful for viewing
GvSig
OpenJump, QuantumGIS, GvSig, uDig