How Many Ways Of Communication Across Long Distances Are There?

[Scorponok]

I can think of Whispering Flame from Eberron, and Dream from core, but after that, I'm drawing a blank. If two civilizations are trying to find a new land to settle, and they split up, is there a possibility they will be able to stay in touch with each other? Caster level of the best magic users is around level-8.

[Bad Wolf]

You could try Scrying. One scries the other, sends a Message spell saying he wants to talk to him, the recipient scries him back, the guy voluntarily fails his save. Voilà, you can see and hear the other person.

[CGNefarious]

Sending should work, though it's generally only for shorter messages. Twenty-five words or less. It's also a fourth or fifth level spell. No maximum distance though, so that's good.

[Allanimal]

Animal messenger might do the trick.

[Fizban]

The best long range communication spells bar none are Whispering Sand (Sandstorm) and Forest Voice (Complete Champion). Both are 3rd level and allow two way communication for minutes at a time with unlimited range on the same plane. Whispering Sand lasts longer, connects multiple creatures, and works as long as the targets are carrying a bag of sand, but you have to know or at least name them. Forest Voice is shorter duration but you can target the nearest matching plant to the coordinates chosen, and you could use potted plants to mimic the other's "target guy I know." Both are druid spells but WS is also a sor/wiz spell.

[Zaq]

Congress of Shadows (ToM pg. 143) is limited to 1 mile/level, but it's worth mentioning because it comes online earlier than most other long-distance communication spells (and spell-esque things). As a 2nd level mystery, it's available at level 3. Sure, that means that it's only good for up to 3 miles as soon as you get it, and only for 8 words on either side, but when you consider that Sending isn't going to be around for another four character levels at the earliest, it's worth mentioning. Not going to get you across a continent (unless it's a very wee continent), but better than nothing.

[Cicciograna]

Aspect Mirrors, Complete Scoundrel.

[Necroticplague]

There's an old trick involving Trolls to deliver messages. The largest part of them regenerated into the creature, as long as it isn't killed (which requires a specific weakness). So if you prep ahead of time by say, tearing off its arm and taking it with you, you can use the creature to send a message by later reducing it until all its parts are smaller than the arm the other party is carrying.

[Flickerdart]

There's an old trick involving Trolls to deliver messages. The largest part of them regenerated into the creature, as long as it isn't killed (which requires a specific weakness). So if you prep ahead of time by say, tearing off its arm and taking it with you, you can use the creature to send a message by later reducing it until all its parts are smaller than the arm the other party is carrying.
This "old trick" has never actually worked - the regeneration rules state explicitly that "Severed parts die if they are not reattached." So if you cut a troll into 1000 pieces it will regenerate from the biggest one, but if you just lop off an arm and take it away, the arm dies and becomes useless for the entire thing.

[Fizban]

Congress of Shadows (ToM pg. 143) is limited to 1 mile/level, but it's worth mentioning because it comes online earlier than most other long-distance communication spells (and spell-esque things). As a 2nd level mystery, it's available at level 3. Sure, that means that it's only good for up to 3 miles as soon as you get it, and only for 8 words on either side, but when you consider that Sending isn't going to be around for another four character levels at the earliest, it's worth mentioning. Not going to get you across a continent (unless it's a very wee continent), but better than nothing.
Ah, forgot about that one, liked it though. Same level as Whispering Wind (PHB) but the message actually travels instantaneously like magic, instead of at a brisk walk like a commoner.

[Telok]

This "old trick" has never actually worked - the regeneration rules state explicitly that "Severed parts die if they are not reattached." So if you cut a troll into 1000 pieces it will regenerate from the biggest one, but if you just lop off an arm and take it away, the arm dies and becomes useless for the entire thing.
Well we can fix that with quintessence. Actually psionics probably works best here. Clairvoyant sense is low enough and lacks a range limit, you just have to specify times and places to leave written messages.

Ring gates, crystal balls, effigy homing pigeons, commoner railguns, bound or allied demons or angels with at will teleportation, paid illithids (plane shift is inaccurate but at will again). Anything that can get to and from the astral, etherial, or shadow planes twice a day.

Almost any divination spell that doesn't have a range limit can be leveraged into a communication system if you can cast it often enough. In theory you could have a world wide communication net using controlled ghouls and resetting traps of cure light wounds, cause light wounds, and circle dance. Each link has two ghouls on each end, all the ghouls know each other, one is the sender whose on/off signal is controlled by the cure/cause traps and the other is the receiver that reports the condition of the sender on the other end every time the circle dance trap cycles. Now just translate your message into binary and send it via the hurt/healthy report of circle dance.