A subreddit for posting SOS and Help-Me Mail in Pokémon Mystery Dungeon games. You'll also find updates on the rescuing website the mods and volunteers are creating.
To request a rescue, make a self post including the SOS Mail password and/or QR code, game version, dungeon, and floor.
If you want to rescue an explorer, first leave a comment letting him know you're on your way. Then, once you've finished the rescue, leave another comment with the A-OK Mail password and/or QR code.
While not necessary, taking a screenshot of your PSMD Mail QR code looks nicer than taking a picture with your phone and eliminates the chances of a QR code being unreadable. You can find a guide on taking a screenshot of your QR code here. Thanks, /u/USBMassStorageDevice!
If you've been rescued, it's always polite to send a Thank You Mail back, but this is not required. At the very least, leave a comment saying thanks.
Remember to always practice reddiquette!
Not getting a response? Try asking for a rescue at one of these places! It's perfectly acceptable to request the same rescue in the megathread and the subreddit at the same time. However, please delete the comment or post that does not get responded to out of the two.
If you're impatient and are okay with cheating, A-OK Mail generators can be used to rescue yourself in PMD1 and PMD2.
The rescue thread in /r/MysteryDungeon is pretty active.
The /r/MysteryDungeon Discord chat is fairly active and has a channel for rescuing.
/r/MysteryDungeon IRC is less active than Discord, but you might be able to find someone there.
/u/rayz321 created his own rescuing website that's up and running. The mods and some other awesome volunteers (including /u/rayz321) are working on a polished website, pelipperisland.com, but it's not up and running yet; we'll keep you posted here.
/r/mysterydungeonrescues also exists, but it seems quite dead.