There are a few websites online that have "Minecraft" block ID lists, the most popular of which is minecraft-ids.grahamedgecombe.com. This website includes a full list of all "Minecraft" blocks and items, along with their ID numbers and ID names, up to date as of "Minecraft" version 1.8.8. There is an additional list of all mobs and entities alongside their ID numbers and names as well.
These block and item IDs can be used to spawn in blocks or items in-game using the command Give. This command is available in single player or LAN games with cheats enabled and also to operators on multiplayer servers. The Give command gives an item to a player, using the syntax "/give player item [amount] [data] [dataTag]". The "player" specifies the target to give items to, which must be the user name of a player currently online on the server. The "item" is where you place the item or block ID.
The ID used should be the number for "Minecraft" versions below 1.8 and the ID name for version 1.8 and above. The [amount] specifies the number of items to give. It defaults to 1 if no value is entered. The [data] and [dataTag] are advanced options for which you can find more information about on the Minecraft Wiki.