This page is designated to database optimization and cleanup tips. As I discover new ones, I will add them here.
CiviCRM_Import_Job Tables - can be deleted after conversion
For those of you who have recently completed a conversion, you may notice that your database now contains a bunch of tables starting with civicrm_import_job_0f214....etc If you browse these tables, you will see that these are the data files that you imported into CiviCRM. Once your imports are completed, you can safely delete, i.e., drop the tables via PHPMyAdmin. Make sure you have a backup before you do this, in case you ever need the history.
CiviCRM_Task_Action_Temp Tables - can be deleted periodically
These are another set of temp tables that are created after a search where actions were taken, such as sending an email, adding a group or running a batch update. These can be deleted periodically as they will accumulate over time.