These are templates that grant some form of respite from aging. Templates with unplayable level adjustments (vampires, etc.) will not be included.
Lich should probably be included. Yes, yes, I know, +4 LA is bad (though not Vampire-level bad) - until Epic. Then it's not so big a deal. Also, if you're going deep into Epic, it opens up the legitimately-good Demilich.
Classes and Prestige Classes
Most immortal character will probably become such because of class choices. For some classes it's impossible to avoid; taking the class to completion grants some form of immortality. Here are all the ones that grant any type of immortality.
Athasian Dragon is missing. Epic PrC, Dragon #339 p.22, grants immortality at level 3. The PrC also has the only means I'm aware of to offload backlash damage from epic spells (to anyone within Close range, and it's blazefire).
1. Unnamed Ring/Amulet/etc. (Dragon 354)
This is the piece of jewelry used in the Kissed By the Ages spell. Any piece of jewelry works, as long as it costs at least 4,000gp and takes-up an item slot. You become completely ageless as long as you wear it and suffer penalties if you lose it. The real advantage to it comes if you combine it with the Item Familiar feat. The ring/necklace/whatever is impossible to destroy without deific intervention and you always know where it is, practically guaranteeing you'll never lose your item familiar and anything you invested in it.
Whether you can make it an Item Familiar is rather dubious, since the spell "forever eliminates any of the item's existing magical properties". You might
be able to re-enchant it (and then make it an Item Familiar), but I wouldn't count on it.
Also, note that Kissed by the Ages targets a person; stealing someone else's KbtA trinket won't grant you immortality.
5. Ioulaum's Longevity (LEF, contributed by pinewoodpine)
This is an epic spell with a Spellcraft DC of 152. It's basically Disintegrate in a 120 ft. radius but your current age category is extended by one year for every living creature it kills. Unlike Thief of Life it doesn't have HD requirements on the target, so (if you don't mind being evil) go raid the nearest village and disintegrate everyone to extend your life. If your morality can't handle that, I'm sure you could get the same results from kobolds (or whatever passes for the "always evil" creature in your campaign). The only difficulties with this are the Spellcraft DC and the fact that you need Epic Spellcasting to cast it. If your DM will let you, drop the Spellcraft DC down to almost nothing by building a crippling XP expenditure into the spell and then using a Thought Bottle (CA) to get it all back. Note that this trick is most definitely not RAI so ask your DM before you try it.
Fortify-based life extension overlaps, rather than stacking (ELH). Ioulaum's Longevity is a bit unusual but the same rules probably apply (it mentions that the age extension is permanent, not instantaneous). This means there's a limit on how much you can get, although the limit is rather large (blasting 5 bat swarms, such as from a vampire rolling high, will give you 25,000 years, and given the 120ft burst area you could fit a lot more in if you tried).
7. Living Zombie Cocktail (CoR)
No, it's not actually called that. Creating a living zombie takes Command Undead, Dominate [type], False Life, Feeblemind, and 1,000gp. You can have one living zombie at a time for every modifier from your primary spellcasting stat (Intelligence for wizards, etc.) which is good because you want as many as you can get. Every one you have makes you age half as quickly as you usually would. This effect is cumulative if you have multiple living zombies, so create as many as you can to greatly increase your lifespan. The only problem is they still age as they usually would so you'll have to replace them every 100 years or so (less if they're elves).
It's fairly hard to crank ability modifiers past +12-15 unless you go deep into epic, so this'll only get you a million years or so.
3. Divine Rank 0+
Almost certainly requiring DM fiat or a seriously high level campaign, somehow obtaining at least divine rank 0 grants the basics of the ideal state of immortality. Note that it is possible to steal enough of whatever the stuff/power is that constitutes divine rank in order to gain 1 or more divine ranks (see Greyhawk sources, the most recent of which is "Expedition to Castle Greyhawk" for details on how this was/can be accomplished. Alternatively, see Forgotten Realms sources regarding "The Time of Troubles".)
Dragon Ascendant gives DvR 0 mechanically, but it's a 12-level PrC, and to get in you have to be a True Dragon of BAB +30 and eat your hoard (which presumably translates to your entire WBL at the time of entry).
The Lich-Queen's Beloved adventure (Dungeon 100) states that Vlaakith's divine ascension method (cast thousands upon thousands of Wishes on the corpse of a deity) will eventually work, but the exact number is unknown. An epic ring of at-will Wish should cost 2,775,400 gp by DMG/ELH rules (crafting it would require 137,700 gp, 512,754 XP and 276 days) and should allow 4,800 Wishes per 8-hour day of spamcasting in the Astral Plane.