Orchestra has recently begun going through a major overhaul of pretty much everything, so for the most part there's a lot of pages missing; if you cannot find a page try appending "1e-" to the beginning of it. (i.e. orchestragame.wikidot.com/1e-gear)
Don't worry too much about the overhauls-the setting will stay relatively static (except for gear availability and psychics, which are being mechanically altered dramatically), and though Orchestra is technically entering a "second edition" it's still pretty much the same as the first.
Orchestra Core is maintained by Kyle's Games. It was released first in August 2011, and serves as the general rules for the Orchestra gaming system.
Orchestra Core also comes with a variety of setting elements required to play Orchestra. These setting elements are called Orchestra, but the Orchestra gaming system is independent of the setting (typically this will be denoted as OrchestraS).
Here are the most recent first edition Orchestra Core files:
Orchestra Core will only occasionally be updated in PDF- the latest version will be here on the wiki for everyone to see as changes are actively being made, with PDF files being reserved for printing.
Kyle Willey of Kyle's Games
Multiple members of the 1km1kt.net forum (Chainsaw Aardvark, Evil Scientist, SheikhJahbooty, BubbaBrown and Onix in particular).
Chainsaw Aardvark and Evil Scientist were generally awesome and helpful, though everyone there is helpful.
Onix is responsible for massive improvements to the scene system.
SheikhJahbooty is to be thanked for Events and getting me back working on perks and maneuvers, and lighting a fire under me to actually work on co-operative rolls.
BubbaBrown was kind enough to make a character sheet, and provided help and inspiration for my development of the final version of the sheet.
Catlike Coding was kind enough to make AnyDice, which I recommend, as well as an Orchestra-specific dice script (http://anydice.com/program/a37). It will work on smartphones and has probability curve information alongside a roller (you will have to add your own modifiers, but that's basic math).