I am looking to get some guidance and feed back from the community as to the prevalence and usage of the following technologies (on SQL Server mostly but answers for any database are very welcom):
ODBC Descriptor support
OLE DB support
External Linked Server support
Data Link support for Data Transformation Services
Specifically I am hoping to find out when each technology should be used and the types of applications that use them. Additionally, I am wondering if there are alternatives to these technologies and why they may have been used instead.