In my intro thread, I mentioned the stuff I do is pretty mundane, some of the projects I've seen here are amazing, while mine are not in that caliber, I going post a few.
1+2) The first is the 1950 era Bridgeport milling machine, that I bought and then found out it had a bad M head assembly. After pricing a replacement, I decided to adapt a 1986 Central Machinery mill/drill head, it has 12 speeds and a R8 spindle. Originally this was a temporary modification, but it has worked so well, at this point until this head assembly fails, it going to stay. The machine is run almost daily and works great. Another benefit, because of the way it is built, the maximum distance between the table and the quill is 36".
3) The Jet JVM 836 milling machine I bought has been great, but was somewhat limited by the 5 spindle speeds that came from the factory (240-1550 rpm), so I made a "transmission" that by increases the number of speeds to 25 (48-1500 rpm), it didn't increase the max spindle speed, but dramatically lower the minimum speed and made the steps between speeds much smaller.
Jack