Only being 19, I don't have too much to boast about, but will give it a shot.
I did most all of the designing and construction of the big trebuchet listed in my local paper, threw a 12lb bowling ball over 400 feet:
Link to online article
The Technology Student Association for Texas now has a new rule in place known as the Canyon High Rule, because of me. I designed the model rocket that the team took to state, and I wasn't even on the team officially. It ran off of a G sized model rocket engine, they are now limited to nothing larger then an A size engine. I had to send them with a larger launch pad to handle the rocket safely, as what they had at state was only setup for much smaller rockets. This was two years ago.
I am currently working on a large Pulsejet engine to be mounted on a gokart, should produce around 210lbs of thrust, and will cost around $70 to run for 15 minutes of pure fun, if I can get it to run.
Not really a great achievement, but how many people on here can say that they can ride a unicycle?
If I stop and thank about it for little longer, I can probably come up with some more stuff that I have done over the years.