I've been very intrigued with plug welds as of late; the way they meld two pieces of metal and with a quick brush with the grinder seamlessly disappear. I subconsciously became aware of this technique as a teenager when working on the frame of my pickup truck. It is commonly used in automobile plants to rigidly join members of a car or truck frame together without hardware.
Later in college, a professor was explaining his fascination with one of Mies van der Rohe's masterpieces, the Farnsworth House, and how he was long perplexed as to how the vertical elements of the exposed steel frame were joined to their horizontal counterparts. Upon further investigation, he learned that Mies had hired an ornamental iron worker, not a structural welder, to perform a series of slot welds along the flanges of the I Beams. The result was a beautifully crafted, extremely strong, and completely hidden joint. Mies famously said, "God is in the details." An illustration of the joint is shown below:
And below is the finished product, the Prairie Table. With a total of 42 slot welds, it was a labor intensive piece. However, it was a great way to dive into plug/slot welding and has taught me many things which I will incorporate in future projects.