Now I know that the IP is my internal IP which only people within my network can join, but I thought, "Can others join too if they had my external IP?" So I portforwarded the port that my local server was using and I gave the IP and port to a friend of mine who was clearly not on my network. Guess what, he got in. I don't know if anyone discovered this already but you can try it for yourself.
How to do this: Just make a Singleplayer world, type /publish, portforward the port it gives you (NOTE: The port is not the same everytime you open the server), get your EXTERNAL IP address on a website like whatismyip.com or ipchicken.com, then finally, give the external IP and port to the person who will join the server.
tl;dr You can let anyone join your LAN server if you portforward the given port and give external users your external IP (whatismyip.com) and said port.
I haven't been back to this thread in quite a while and I see that some people are confused with the process. Portforwarding differs from router to router. I'm not very good at doing it to routers other than my own. If you need steps on this process, there are many tutorials on portforward.com. Also, MineCrak has a link to a topic with instructions for using Hamachi.
btw: Ivorius has just posted detailed Instructions for all platforms on how to extend your LAN-Server to others over the internet via Hamachi.
So two ways to get the job done. I've also posted a back link in that thread to this one, so the circle is now complete.