For all practical purposes, there are no DBMS vendors left advocating single-server strategies. Oracle was the last one, but it just acquired in-memory data management vendor TimesTen, which will be used as a cache in front of high-performance Oracle databases. (It will also continue to be sold for stand-alone uses, especially in the financial trading and defense/intelligence markets.)
IBM’s Viper is a server-and-a-half story, with lots of integration over a dual-server (one relational, one native XML) base. IBM also is moving aggressively in data integration/federation, with Ascential and many other acquisitions. It also sells a broad range of database products itself, including two DB2s, several Informix products, and so on.
Microsoft also has a multi-server strategy. In its case, relational, text, and MOLAP storage are more separate than in Oracle’s or even IBM’s products; again, there’s a thick layer of technology on top integrating them. An eventual move to native XML storage will, one must imagine, be handled in the same way.
Smaller vendors Sybase and Progress also offer multiple DBMS each.
Teradata is a pretty big player with only one DBMS — but it’s specialized for data warehousing. Teradata is the first to tell you you should use something else for your classical transaction processing.
The Grand Unified Integrated Database theory is, so far as I can tell, quite dead. Some people just refuse to admit that fact.