Windows only: You've probably tested your internet connection speed (I've always preferred Speakeasy's Speed Test) and have a pretty good idea of your broadband speed, but free Windows app LAN Speed Test measures the speed of data transfers on your home network.
If you're sharing files or streaming videos over your local network, your network speed is a pretty important factor to keep an eye on, and if you feel like your network's not up to snuff, a quick test may help you figure out if you need to replace or upgrade your router. LAN Speed Test tests how quickly data can travel between computers over your network. It takes a little figuring out to use, but how-to blog MakeUseOf has instructions to get you started. MakeUseOf demonstrates how to use the completely free 1.1 version, but the application also has a newer version that has limitations unless you pay.
LAN Speed Test is a free download for Windows only.