If you read my post on Metadata & Your Content, then you have an understanding of what metadata is, now you will learn about three companies that specialize in compiling metadata. The three companies are Echonest, Gracenote, and Rovi. This isn't an in depth post about the companies, it's more of a who they are and how you can get your content metadata into there database.
Echonest collects metadata for music only (as of now). It is true what they say, do one thing, and do it well. Echonest focus on music has allowed them to create an amazing music metadata hub. Echonest doesn't have a content submission section, but a representative of Echonest sent this explanation of how their platform discovers your content – “We currently don't offer a way for artists to submit music directly. However, if you have recordings on the web or through digital retailers, our platform is designed to discover new artists as they release music organically through web presence”.API available to developers: Yes
Gracenote collects metadata for two categories of entertainment, music and video. If you use iTunes then you've interacted with metadata supplied by Gracenote. Gracenote doesn't stop because they supply iTunes with metadata, they also supply in car infotainment centers with metadata. Some of Gracenote platforms are, Music ID, Scan & Match and Gracenote Powered Mobile Apps. You can submit your metadata directly to Gracenote.Gracenote content submission information
API available to developers: Yes
Rovi is a metadata service that compiles metadata for multiple intellectual property categories – music, television, video/DVD content and books. You can submit your metadata directly to Rovi.Rovi content submission information
API available to develpers: Yes