How to interface with relational DBMS like PostgreSQL, MySQL, Oracle?
Check out SqueakDBX, a project that uses the open-source C library OpenDBX to interact with major relational database systems. There are also interfaces to NoSQL DBs, like Cassandra and CouchDB. You may also want to check out the Smalltalk object DB Magma. The GemStone solution GLASS, on the other hand, provides a whole stack including Seaside.