The "/var/crash" directory holds system crash dumps according to Filesystem Hierarchy Standard (FHS). You may get reports on a system's boot when crash report files exist. When the "/var/crash" directory contains files, Apport will display notifications. This directory is then cleaned up by Apport using "/etc/cron.daily/apport" job. Read more: [...
journald is the part of systemd that captures, queries, and removes logs when needed. It allows removing logs by setting "--vacuum-time". systemd is also known as "System and Service Manager" and is a suite of basic building blocks for a Linux system. systemd keeps files in two places: "/var/log/journal/" (persistently) and "/run/log/journal/" (volatilely, the data is lost when the system is rebooted).
Zeitgeist logs files opened, websites visited, conversations, and emails and provides this information over an API to applications. It serves as a comprehensive activity log and also makes it possible to determine relationships between items based on usage patterns. This script deletes indexes ("fts.index" directory). It's safe to delete and will be regenerated the next time you start Zeitgeist. This script also deletes all activity logs ("activity.sqlite") and related user-data.