List of sprite-based sideview systems here:
Cybersam's V 2.5 and 3.0beta in the [downloads] section
Minkoff's Animated Battlers (I've been maintaining)
Trickster's Multi-View (being reworked)
Ccoa's Sideview System (a true CBS)
Most of the sideview systems attach themselves TO a frontview system. They are not true sideview systems themselves, but rely on a battlesystem... whether the default system or an advanced battlesystem.
Trickster's battlesystem is being reworked and not really available right now. When it is... ow!
Ccoa's sideview system is the only battlesystem that has its sideview system built-in.
The rest is sprite-editing, background artwork and sideview-system configuration.