I have my computer setup to dual-boot between Ubuntu 14.04 and Windows 8.1. I needed to transfer files between OSes but I didn't want a 3rd party file explorer to do so. So, after some investigation, I found a driver that allowed the built-in Windows Explorer to read ext2/ext3/ext4 partitions, which is what Linux uses.
Step 1: Install the Driver
First off, you'll want to download ext2fsd, which is the driver that enables Explorer to mount ext partitions. You can get the latest version (at the time of writing) from here.
Once downloaded, open it up, and just click "next" until it's done.
Step 2: Mount the Partition(s)
When you open ext2fsd, you'll be presented with a list of all the partitions on all the drives present on the system.
In Ubuntu, I have two partitions, the system one, and the user data one. Here I mount both. To mount a partition, just double click it in the list, select "Mountpoint for fixed disk, need reboot" and select the drive letter or number you want to assign to it. I assigned U to the user data partition and S to the system partition.
Step 3: Reboot Computer
Once you've chosen which partitions to mount, you'll need to reboot the computer so that the changes take effect.
Step 4: Ext Partitions Now Show Up in Explorer
Once your computer rebooted, click on the Explorer icon and you'll see that the ext partitions are listed as normal drives.