Updated guidance for users of Windows 10 1809 or later:
In Windows 10 1809, we (finally) shipped filesystem integration allowing you to access the files in your WSL distros from Windows via the \\wsl$
UNC path:

If you're interested in the details behind how this works, please visit this blog post: https://devblogs.microsoft.com/commandline/a-deep-dive-into-how-wsl-allows-windows-to-access-linux-files/
This now allows wslpath
to provide a Windows-accessible path to files within your distro's filesystem:

In the up-coming Windows 10 May 2020 Update (2004), you won't have to remember the somewhat obscure \\wsl$\
UNC path - instead you'll just click on the Tux (Linux penguin) icon in your File Explorer:

Warning for WSL1 users:
Do not try to spelunk to the %localappdata%\lxss\...
folders containing your WSL1 Linux files from Windows - there be dragons!
Please read this post: https://blogs.msdn.microsoft.com/commandline/2016/11/17/do-not-change-linux-files-using-windows-apps-and-tools/
This guidance has not, and will not change, though it will become less important over time: In particular, WSL2 does not use %localappdata%\lxss
- instead it stores your linux files within EXT4 formatted VHDX files delivering near-native IO performance for the distro's local filesystem.