ARM architecture development kernel - a consolidation of various new ARM architecture features merged into a single tree on the masterbranch. The patches are also available on individual arm-arch/*branches. The tree is updated every 1-2 weeks following mainline -rcreleases.
ARM Embedded Linux (AEL) - Linux kernel patches, config files and pre-built images for ARM Ltd. platforms (usually updated twice a year).
Catalin Marinas' kernel tree - Miscellaneous fixes (master branch), ARM LPAE (for-next branch) and kmemleakpatches. The first two are also available in the ARM architecture development kernel.
Will Deacon's kernel tree - several branches, most of them also merged in the ARM architecture development kernel.
Marc Zyngier's ARM platform consolidation kernel tree -various branches for ARM architecture developments (generic timers) and platform consolidation. Most of them are also merged into the ARM architecture development kernel.