YaBB developers & BoardMod contributors, Jet Li and Deti release another new beta mod called... Reverse Post in Topic Adv. Ed. V1.0 for YaBB 2.2.1 .  Here are the details as posted by Jet Li on BoardMod:

Description:
With this Mod Admin can choose default post-order and can allow or not the individual choice of post order. If allowed each user can change the order of the posts by clicking on the name of the topic on topic pages and/or set it in Profile - Options

Features:
- Enable/Disable Reverse Topic Summaries in Topic, so that latest post is at top (default for all members/guests)
- Allow each User can Reverse Posts by clicking on the name of the topic on topic page and/or set it in Profile - Options