Lego is maybe the most well-known brick building system in the world, but when the patents expired, the clones came to market and lawsuits would follow.
I'm going to combine my Tyco military set with my Lego Titanic model and make a really cool naval battleship scene.
I don't remember the LEGO battle with Tyco, but I do remember receiving Tente blocks from a well-meaning relative. They were smaller and incompatible with LEGO. I think I had a battleship and maybe an aircraft carrier. They were fun, but you had to be careful not to mix them into your LEGO stash.
Dollar General, Big Lots, and a few other discount stores have knock-off LEGO-but-not-LEGO sets. I don't know what the quality is like, but they are always priced 1/3 the cost of official LEGO sets which I'm sure makes them very appealing to parents and grandparents.