I was wondering, how can I make a never ending fire in Minecraft? (I want to use it in SMP)
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.
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.
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.