WARNING: Some SystemUpdaters may not be compatible with any 3DS and can cause bricks. Warnings are provided where possible, though I cannot guarantee the safety of your consoles/devkits, and I take no responsibility if anything provided causes any damage. It is up to you to check everything before using.
Official changelogs for every version of the SystemUpdater and SDK dating back to 0.7.0 (2009/12/22).
Official readme of the 0.35.0 SystemUpdater, currently the latest version.
Unofficial list of all known SystemUpdaters. This has info on whether or not a given updater has been dumped/leaked.
Downloads of most SystemUpdaters known to have been dumped/leaked, along with the extracted firmware CIAs which have been retail encrypted to work on normal peasant retail units.
Note that this does not include prototype (<0.13.0) or ntrboot mode SystemUpdaters.