The Structure API provides information about many of the new structures to apps and websites like Evernus and jEveAssets. CCP's APIs only provide structure information when you have docking access to the structure, but its not uncommon for players to find that they have assets, wallet transactions, contracts and so on at structures that they can no longer, or were never able to dock at. This API provides a superset of the information that CCP provide through ESI and is augmented by players who bookmark the structures to claim bounties as they explore space. If you wish to contribute to this API (perhaps to have your citadel assets show up properly in jEveAssets or to top-up your income from exploration), read the Getting Started Guide. For more info about this API, reach the Structure API documentation.
The System Index Multiplier API assists with the planning stage of industrial production or research, by doing the hard work from Appendix 2 of the Eve Industry - all you want to know dev blog (which, we discovered, has missing and incorrect numbers). More info at the System Index Multiplier API documentation page.
Use Accept Encoding: gzip if you're mass-downloading data.
The Cache-Control: max-age header is set on all responses so there's no point polling these APIs more often than the value returned in that header.