This tool lets you quickly subnet IPv4 networks, visualise them, and split or join them with a single click. It also shows the address range, subnet mask, and number of hosts for each subnet. You can save and load your arrangement using bookmarks.
Access the Visual Subnet Calculator.
This was written in 2002, though I still use it constantly. One day I intend to make an equivalent for IPv6.
Update 20/9/2017 - the source is now on GitHub under the zlib license.