Fallout veterans know about the "Alien Blaster," a special (and powerful) weapon that's made an appearance in every Fallout game in the franchise to date. Here's a video guide on how you'll find it in Fallout 4. Written instructions below, too!
First, head to this location on your map. It's right by Oberland Station, if you're having trouble locating it:
Then, turn around and walk past the broken fences, into the woods. Keep going. Eventually you should come across the wreckage of a ship:
Hop down, and you should spot some green goop:
Follow that trail of gunk. It should lead you into a cave. Inside, you'll find an alien -- turns out, that goop was blood! Well, you're here for a reason, and as sucky as it is to say, you're going to need to kill that poor bastard. Don't worry, the alien was pretty near death anyway.
On that alien's body you should be able to find this goody:
Along with a few hundred shots for your gun. That ammo is limited! But, you'll note, it's possible to modify the weapon so that it accepts Fusion Cells instead of the other, rarer ammo. It's a nice option, though you'll sacrifice some damage to do it.
If you're a visual learner and want to actually see exactly how to get this weapon, make sure to watch our how-to guide video at the start of this post. Stay for the end, too. You'll see a mini-montage of this gun wrecking some enemies in action, which will give you a sense of what makes this weapon so damn special. Not only does the Alien Blaster look awesome, it also vaporizes your enemies in a neat way!
Note that this is an event in Fallout 4, and there's a chance it might not occur when you go looking for it. If that's the case, come back later sometime -- it might be there next time.