Well, okay, 'ere we go.
In no particular order:
--The prior codices. The prior Ork Codices has a lot more fluff and a number of characters that were removed from play--from the 4th ed. there was the other named Warlord, Nazdreg Ug Urdgrub. Heck, there were whole units removed like the Skarboyz and Trukk Boyz from 4th ed.
--Sourcebooks. 'Ere We Go, Waaargh the Orks
, and Freebooterz
did not always have rules, but had a lot of fluff and bonus stuff. Zodgrod Wartsnagga, who was a special Runtherd character and Makari, Ghazghkull's Gretchin banner waver showed up in there. (And they have their own minis, too.)
--White Dwarf. There are a lot of issues that have Ork fluff. For instance WD389 and 390 (June and July of this year, respectively) has a battle report on Orks v. White Scars v. Necrons. WD259 has the Feral Ork Codex (and it's Chapter Approved). I like it since it has rules (albeit outdated ones) about fielding a Squiggoths in the Heavy Support slots in non-Apoc levels. Find a store that sells the old issues and just leaf through them.
--Gorkamorka. The now defunct game had the following sourcebooks: Gorkamorka, Da Uvver Book,
. It also had its own line of minis. The Orks are smaller in stature (more along the lines of a yoof, than a full-grown boy). And there were different looking vehicles (their trukks are more like the warbuggies in the present line).
And then there's the unofficial stuff like Citadel Journal,
which was a fanzine and the RPG stuff from Fantasy Flight Games (one of the Deathwatch expansions had a profile of Big Mek Buzzgob, who also appears in Forgeworld).
And I think that's about it. *Goes back to check OP.
* For a lot of info (that may or may not be canon) check out sites like the Lexicanium. It has a lot of info on all things 40K (and FB, since there is a sister site).