Optimize › Tables lists every table with its rows, size, overhead and engine. Optimize and repair tables, and convert MyISAM tables to InnoDB.
Tables left behind by removed plugins are flagged. Only tables that no installed plugin or theme claims can be emptied or dropped, and you type the table name to confirm. WordPress core tables are protected.
Scheduled events
Tools › Scheduled events lists every cron event with its next run and owner. Run one now, delete it, and spot events that may belong to plugins you removed.