I was wondering, how can I make a never ending fire in Minecraft? (I want to use it in SMP)
You need to use a Netherrack block, as all other blocks burn up or won't catch on fire.
Nethererrack burns for ever.
Prior to the 1.3 patch, you could make an infinite fire by lighting a block of wood / log on fire so that only one side is burning, and it would stay infinitely burning. However, that was fixed in 1.3, so now the only way of having a block infinitely lit on fire is to get Netherrack somehow (be it buying it from an in-game shop, mining Coal Ore with a Golden Pickaxe if your server has the Golden Nether mod, or simply going to the Nether if your server supports it) and lighting it.
There used to be a bug that allowed you to do infinite fire, however, it was removed.
Now the only way is to harvest Netherrack from the nether. Then you can use flint and steel to light it once you have placed the netherrack down.
In SMP you would need a nether world or use the /give command.
You could use a netherrack block. Just mine it from the nether.
Alternatively, you could use
/give [user] 87 to get yourself a netherrack block.
This would be the only solution other than using a plugin.
Netherrack fires never burn out and if you want to read about them, check the Minecraft Wiki, Netherrack article.
/gamerule doFireTick false will make all fires burn infinitely without spreading.
You can harvest netherrack and use that, but unless you want a fireplace why get a never ending fire? Also, light the netherrack with a fire charge/flint and steel.
You need Netherrack, a block that makes fires burn unlimitedly.
Netherrack can be found in the Nether, a dimensions that can be accessed by a portal built by 4 by 5 obsidian blocks. But keep the inner blocks empty. Then light the obsidian with flint and steel and the portal should be active. The Nether is full of Netherrack, so it's easy to collect. Once Netherrack is lit, the fire burns forever.