I guess nxMyDB and ioShareDb are similar in concept, but their designs are quite different.
nxMyDB uses MySQL's GET_LOCK/RELEASE_LOCK for locking. ioShareDb uses site ID+locked column (which worked poorly, especially when the locked flag wasn't cleared).
nxMyDB uses the MySQL client library directly. ioShareDb uses ODBC.
The list could go on, but you get the idea.