Viewing the world seed in the multiplayer mode

I was able to find out the seed for my multiplayer world by downloading this program and opening the level.dat, with the program, in the world folder for the world that you are running. When you do that, it will display the seed in the top-left corner of the program. To make sure that it is the correct seed, simply make a new singleplayer world with that seed and check the coordinates by pressing F3. You can then go to your multiplayer world, to see if the world looks the same by going to the coordinates that you checked in your singleplayer world.

Enter the /seed command in chat and the seed will be displayed. In vanilla servers, operator access is required to use this command.