FlashMe v8a
(official download page)
Warning! DS Link is reported to have problems with this version.

Normal version : Boot-up logo and safety warning are removed.
"Stealth" version : Warning screen is left intact.
NoFlashMe : Uninstaller

Other versions
No autoboot (stealth) : "PASS" check is removed. SLOT-2 is only booted when SELECT held.

Run flashme using some device made for running homebrew code. Follow the on-screen instructions. The SL1 contact (pictured above) needs to be shorted with a small metallic object like a small flathead screwdriver, bent paperclip, etc. Be careful not to touch any other parts inside the DS, or it may shut off unexpectedly.

To upgrade to a newer version, just run it. You don't need to uninstall the previous version.

Run noflashme to completely remove flashme.

If DS software is detected in SLOT-2, flashme will automatically run it. Otherwise, it will launch the normal DS firmware. To override this behavior (always boot to the firmware), hold the SELECT button when you power on.

To force it to boot from SLOT-2, hold A+B+X+Y.

(no-autoboot version): SLOT-2 is only booted when the SELECT button is held.

Failsafe code is written into the first few blocks of firmware, in case the flashing procedure gets interrupted or malicious software overwrites part of your firmware. If your firmware gets corrupt, hold START+SELECT to boot directly to SLOT-1 or SLOT-2. From there, you can run flashme or noflashme to repair it.

For more information, refer to this wikipedia article.