[3.5] Ways to learn spells from a different class list

[Clistenes]

I would say that, using the method of research of new spells described in the DMG I think you could:

If you are a cleric, learn any druid spell at the same level.

If you are a druid, learn any cleric spell at the same level.

If you are a cleric, learn your domain spells as normal cleric spells with level +1 (for example, a 5th level domain spell can be learned as a 6th level cleric spell).

If you are a cleric, you can learn a wizard spell as a cleric spell with level +2 (I have taken Antimagic Field as inspiration).

If you are a wizard, you can learn a cleric or druid spell with level +2 (except cure spells or heal).

Of course, it's just my opinion, and not an official rule.