NDS:FlashMe

Loopy (Olimar)'s FlashMe is the best known of the custom firmwares for the DS. The current version, v8a, fully supports the DS Lite - including uninstallation.

Features
While primarily invented for the convenience of Slot-2 flashcard users as an alternative to carrying a PassMe/NoPass (and therefore of reduced value nowadays), some of the side features are still of interest:
 * Optionally skipping the startup animation (with a non-stealth version).
 * Accepting unsigned software over DS Download Play.
 * Anti-trojan brick protection (by stuffing a basic card loader in the write protected area).
 * Allowing variable brightness on all consoles capable of it.
 * Updating your DS in the progress, not that it's useful ;)

The "noauto" variants, which omit Slot-2-as-DS booting by default, are recommended for most users.

Installation
Installation simply involves running the installer and shorting the SL1 pins (which disable write protection), conveniently accessible from the holes around the battery compartment after removing its cover, facilitated in the newest versions by added audible feedback. However, especially with the tighter packed DS Lite, care must be given to not short SL1 to other parts, especially VR1/VR2 (the potentiometers for screen bias voltage adjustment, also accessible in the same manner), as a likely consequence is causing the console to power off in a most inappropriate moment.

Should it be needed, the firmware (which is part of the Wi-Fi card) can be programmed via a parallel cable, or the card can be hotswapped with a known working one on the DS Lite.

Alternatives
Other, less popular third party firmwares include FWNitro, Nocash's (part of NO$GBA's Xboo solution), CreeboMe, and CTurt's. Not all of these are DS Lite compatible and few would recommend them, but are mentioned as further reading for the enthusiast.

Source code
The source code for FlashMe can be found here: https://gbatemp.net/download/flashme-source-code.38064/