You can create an infinite well using two buckets of water and a hole that is one block deep and two blocks by two blocks wide. I'm wondering if it's possible to do a similar thing with lava, now that obsidian is so valuable for making portals. I have an idea for making it easier for creating portals, but it's dependent on having an effectively infinite source of lava very close to where I want to create the portal. So is this possible? If so, how?
Not Any More
In Minecraft Beta 1.9 pre-5, you could make infinite quantities of lava by placing 4 lava sources around a central block as so:
As pointed out by Ben Blank, this behavior was scheduled to be reverted, and no longer works.
Both top answers rely on bugs that existed in previous versions of MineCraft, but have since then been fixed.
In the current version (MineCraft 1.7.4), there is no way to get infinite lava source blocks or lava buckets.
However, you can get infinite obsidian by exploiting the fact that portals between the nether and the overworld are re-created (offering "free" obsidian) if no existing portals are in range. This mechanic is described about half way through this video, and still works in 1.7.4:
There is no way to replicate lava, like water, in actual Minecraft.
However, I found a way to get lava infinetly. In the Nether, the netherrack walls contain lava patches. I also noticed that netherrack walls regenerate if you go too far away from the mined channels. So, lava might also re-generate in them.
I'm playing the newest update in multiplayer and I've noticed that while farming cobblestone, sometimes a lava flow block turns into a source block when the surrounding flow blocks turn into cobblestone.
My hypothesis is that this has something to due with the lag (I play on a very busy server). I play on a Skyblock server so being able to duplicate lava source blocks (even if the result is uncommon and unpredictable) is very useful because of the limited resource availability.
A top view diagram of my cobblestone farm:
X XX#XX Xx xX Xx xX XX@XX X
x- 1 stone block
X- 2 stone blocks (stacked);
@- Water placed on a stone block (after farm is filled with flowing lava);
#- Lava source placed on a stone block