Castbar Mods

AdiCastBars

AdiCastBar is a small, simple casting bar addon. Features: Replace Blizzard casting bar, Player casting bar with integrated latency and pushback display, Big target casting bar, Global cooldown display, Bars can be moved and slightly configured using the /adicastbar chat command, Alternative bar texture can be selected by using SharedMedia and !SurfaceControl, ConfigMode support.
Curse

CastBars

Castbars is a lightweight enhancement of the Blizzard Casting Bar and Mirror Timer Bars. Castbars works by modifying the existing Blizzard Casting Bar and Mirror Timer Bars and makes heavy reuse of the existing Blizzard code for driving all frames.
Curse
Review: WoW Insider

Gnosis

Gnosis is a highly configurable castbar addon, but featuring much more than simple castbars. Gnosis can show ticks of channeled spells, detect unintentional clipping while channeling and combine data of channeled spells to combat text addons (MSBT, SCT, Parrot).
Curse, WowInterface
Review: WoW Insider

Quartz

Modular approach to a casting bar addon. An overview of (hopefully most of) the modules: player, target, focus, buffs, flight, global cooldown, interrupt, latency, mirror, range, swing, timer, tradeskill merge. The core of Quartz is lightweight implementation of a standard casting bar, with configurable size, text and icon positioning, and colors.
Curse