Recently, I purchased a Hoya Polarizing Filter, and wow, my. life. is. changed.
Here you can see what happens without a polarizing filter when photographing a reflective surface:
And here you can see what happens with a polarizing filter:
Ok, so it's not the prettiest shot ever, but the dirt on the windshield and the dimmed view through the vehicle was impossible to see without the filter. With the filter, the glare is drastically reduced, and there's a lot more detail in the shot.
So to get those dreamy terrarium photographs where you can actually see everything inside the terrarium, use a polarizing filter. To get those gorgeous sky shots with clouds and a lovely blue, same deal. To reduce glare on glossy leaves, yep, you guessed it, polarizing filter. Why didn't I discover this sooner??? BAAAAH!
Now I can't find the webpage that helped me make my filter purchase decision, but if you're wanting to know more about filters and whether you should get one, run a google search. I got good results by searching for: what filters should a beginner photographer get. I got the polarizing filter first because that's something you just can't fake in processing - shots lose unrecoverable detail and information with glare. I also bought the 67mm filter and step up rings based on the recommendation on the webpage I can't find right now.
What filter should I get next? I'm thinking about a warming filter now that my curiosity is piqued. This is my first time using a filter on a camera, and yes, I'm an idiot. SHHHHH!