It occurs to me that there are a few ways to go about having a character 'forever' barring violence.
We have the undead, but arguably its cons outweigh the pros. I'm thinkign about Lichdom and Vampirism here. I don't know enough about Briarhearts to include them. You get to live theoretically forever, get enhanced abilities, and it will generally take much more than a fall down some stairs to kill you off. Yet, you are also dead, will never be able to truly fit back into human or mortal society (if you wish to). Its more suited to the power hungry sort, either way, and with Vamprisim is often seen as either gift or curse.
Binding to a Daedric Prince has some chance of this, but really, there are so many ways this can go wrong or turn out cruelly ironic that it probobly shouldn't be included.
Then there is Corpus, which doesn't bear mentioning, unless you happen to be the Nerevarine in which case you get the one-time-only version that gives you agelessness and immunity to disease. But some day you will still perhaps be foiled by some stairs. Or nick something vital in a fight.
Ulfgaar the Unending is also an interesting one. A spell that turned his pals into stones, but seemed to turn him into a 'human' stone, weathering time since the days of Ysgramor and never aging another day. His is one of the most interesting methods, in that its something that would be priceless if a mage could harness what went wrong as a positive effect.
Thoughts? Any more methods anyone can recall?