All game assets are public domain licensed, which means they're completely free to use in both personal and commercial projects. Attribution isn't required.
Those who have downloaded an asset can give it a rating (5-star system) and optionally a written review. Packages with at least four show their average rating, and total number of ratings. (Packages below the threshold show "Not enough ratings" instead.)
Though assets may be browsed through the web, they can only be downloaded from within a licensed and registered copy of Unity3D (Free or Pro). However, once downloaded, the Asset Store EULA does not prevent using them in non-Unity projects so long as those projects match the "games and interactive media" description.
The Unity Asset Store is well-presented, with an intuitive category system and effective search. In addition, the requirements to place an asset on the store include providing several standardized graphical and descriptive elements so that individual packages match the same presentation standards as the store itself.
The curator of OpenGameArt acknowledges that the tools for finding the assets you need are sub-par. Correcting this is a near-future priority for improvement, and as of September 2014 is the next milestone goal of the site's Patreon campaign.
OpenGameArt hosts art under any of several licenses, some of which may not be suitable for commercial projects or for release on certain distribution platforms. Careful review of which licenses are compatible with your project is essential. The site's search facility allows filtering by license.
Freesound permits uploaders to select the CC-BY license, which disallows the use of DRM, and the CC-BY-NC license, which disallows commercial use entirely. Licenses are prominently displayed on an asset's details page, and represented by a small icon in list views, but there is no option to filter by asset when searching or browsing.
Each sample has a details page which includes a waveform view, a full-length audio preview, user-assigned tags, author-assigned "pack" grouping, and an author chain ("remixes and sources") for derivative assets.
Currently personal storefronts / seller profiles have a simple list of assets. There are no categories, tags, genre-information, type information, etc that could help narrow the list down.
Mobile Game Graphics offers hundreds of free game assets. Get free game graphics, sprites, tile sets, parallax game backgrounds, themes, pixel art, sprite sheet, and characters.
Hosted tracks are permitted to enforce a Freeware license, which restricts their use to freeware projects only. The search system allows filtering by license. Track listings are visually tagged with available licenses.
The Scirra store is particularly easy to use for asset vendors - an approved vendor can upload a new asset in a matter of minutes, version and sub-version handling is handled simply, and demoing and feedback of products can be handled by Scirra's existing (and also very easy-to-use) infrastructure, such as the Arcade and forum. The review process for assets is very efficient, despite ensuring a high quality of content.
Scirra uses a proprietary license for their store, which disallows using any asset in more than 1 commercial project unless the license is re-purchased. Free projects are excluded from this restriction, and it does not apply to assets purchased under an exclusive license.
Best for RPG Games. If you are creating your own RPG or isometric games, Super Games Asset store has the highest quality 2D games assets you can buy online. They offer awesome RPG game icons (these are probably the best you can purchase directly online), 2D sprites, character animation sprites, and huge hand painted RPG game maps in isometric view. Majority of game assets here are visually consistent.