Adobe Mixamo is a website where you can download character models and animations for free (after you create an account which is also free). According to the FAQ you can use both characters and animations royalty free for personal, commercial, and non-profit projects including:

Incorporating characters into illustrations and graphic art
3D printing characters
Creating films
Creating video games

This page explains how to download Characters and Animations from Mixamo and import them into Unity.

Characters

The first thing you want to do is select a character model for the animations to be played on. If you already have a character model you can use the the Upload Character button to use it when previewing animations on the website. Otherwise you can choose one of the available characters then click Download and use the following settings to download the file:

Option	Selected	Reason
`Format`	`FBX for Unity(.fbx)`	Does not actually seem any different from FBX(.fbx) when downloading a model without animations, but you might as well use the option specifically for Unity.
`Pose`	`T-pose`	Does not usually matter because you will generally be controlling the character with animations at all times, but just in case something prevents that from working it would be nice to clearly see that nothing is playing at all rather than having the character in some other default pose which could mean the animation is frozen for some reason instead.

Once you have downloaded a character, you can Import it into Unity.

Animations

Once you have selected the Character you want to use, you can go to the Animations tab of the Mixamo website and find any animations you want to download. You can use the parameters on the right to customise the animation then Download it with the following settings

Option	Selected	Reason
`Format`	`FBX for Unity(.fbx)`	Rather than naming the file AnimationName.fbx, this option will prefix it with the character's name (CharacterName@AnimationName). This naming convention allows Unity to import the animation with the correct name by default. See Extracting Animation Clips for more information.
`Skin`	`Without Skin`	Downloading the Character model separately allows you to reduce the animation file sizes using this option to avoid including another copy of the model in every animation file as well. On a `Generic` Rig this would prevent Unity from displaying anything to preview the animation in the Import Settings (so you would need to select one manually), however using the naming convention explained above and importing the animation file into the same directory as the model allows Unity to automatically use that model for the preview.
`Frames Per Second`	`30`	Setting this to 60 can improve the animation quality a bit at the cost of larger file sizes.
`Keyframe Reduction`	`None`	Can be used to reduce the file size at the cost of animation quality.

Once you have downloaded an animation, you can Import it into Unity.