The resto/feral hybrid. You don't want to play a tank main with a druid, but you can be a healer+offtank with one spec and not really lose anything significant from either role. There's a fully viable raid spec that is both a tank and a healer, thanks to Heart of the Wild in the feral tree which gives +20% intellect. This is a viable laternative to deep resto. The spec looks like this:
http://www.wowprovider.com/?talent=1121 ... c555e501d0If you look at what you give up in the resto tree, it's really not much. Some aggro reduction, some crit on Regrowth which you shouldn't be casting anyway, Swiftmend which is a weak 31-point talent anyway, and then the only significant loss is the +10% healing. I think +20% intellect easily makes up for this.
This build is an incredible dungeon tank, better than a warrior due to its rock-solid AoE threat and very high armor/health. You could also main tank some easier raids, and raid content that you outgear, but it's not really meant for MTing in general. It's a very good OT, however. This spec is not a half-assed healer, it's a 100% capable healer that can also tank with a gearswap macro and do it pretty much as well as any feral tank.
This is the closest thing to dual spec that you'll find in vanilla. The uninformed will look at the 0/30/21 distribution and decide "that can't heal" simply because the majority of the points are in feral, so the spec never gained widespread popularity. However, it was a recognized spec back in the day, and completely viable.