Where can I find information on whether which databases are using B+Trees over B-Trees for their index implementations?
Oracle appears to be using B+Trees. Although they don't describe it on their documentation, their graphics appear to be pointing that B+Trees are actually being used.
For SQL Server the info is here: http://msdn.microsoft.com/en-us/library/ms177443.aspx
In SQL Server, indexes are organized as B-trees. Each page in an index B-tree is called an index node. The top node of the B-tree is called the root node. The bottom level of nodes in the index is called the leaf nodes. Any index levels between the root and the leaf nodes are collectively known as intermediate levels. In a clustered index, the leaf nodes contain the data pages of the underlying table. The root and intermediate level nodes contain index pages holding index rows. Each index row contains a key value and a pointer to either an intermediate level page in the B-tree, or a data row in the leaf level of the index. The pages in each level of the index are linked in a doubly-linked list.
Wikipedia lists a number of databases which support B+ trees.
Note, however, that it is entirely possible for a database to support multiple kinds of indexes