Here are some of my favorite online resources and websites. I have a lot more to add so keep checking back!
Gordon Haine's infamous IX Registry. Although much of the information is quite old and out of date, it's still a good resource and worth a look.
thebmw325ixclub Yahoo Group - pretty active discussion about the ix. Lots of information if you're patient and willing to search. Definitely worth signing up, and a great place to buy & sell ix parts and cars.
Biggest E30 specific forum - best for swaps and suspension/tires.
If you want to turbo or do Megasquirt on your E30, go here.
Largest gallery of E30 cars, euro or otherwise. Lots of information too.
Wikipedia article on the E30. Some ix information.
ETA specific site, but lots of specs and information pertaining to all E30s.
E30 convertable site - invaluable information on power top repairs, as well as all the cool Mtech options that were available.
E30 and E36 318is forum - these guys might be more obsessed over their specific model than even us ix guys! :)
MS1, MS2, and MS3 manuals and forums. Speclizing in the /Extra firmware, MS2 and 3 hardware.
Bowling and Grippo official Megasquirt site. Basic MS theory, manuals, etc.
Jean Bélanger's site, source of lots of excellent add on boards for MS, as well as the must-have JimStim.
Phil Tobin's software site - home to Megalogviewer and Tunerstudio, also must haves!
The best source for MS hardware and information. Highlights are the articles section and the DIYpnp for Motronic 1.3 cars
This is the source for the famous "GG idle board", which enables the use of the stock 3 wire PWM Idle valve for those of us who aren't EE majors. It also has convenient space for 2 additional coil drivers.
Stan Weiss's EFI fuel injector flow data table - flow data and information on nearly every fuel injector made.
Headers & Exhaust Tuning
If you are even *thinking* about doing an exhaust modification to your BMW, read this article first. A fantastic article that explains how to setup a free-flowing exhaust. Debunks the backpressure myth, lots of info on properly sizing primaries, secondaries, merges, exhaust and mufflers. It explains why the exhaust past the final X or Y pipe does not produce power, but only restricts the engine and how to avoid bottlenecks. We can learn something from those domestic guys after all!
Gustave's famous E30 M3 - this is where I started my search into a quiet, lightweight, freeflowing exhaust. Many concepts apply to any BMW. Plus the whole site is filled with great articles and information, I'm glad it was archived!
A basic article on headers and how they work. Clears up the "backpressure = torque" myth that just won't die!
Burns Stainless makes some of the nicest merges on the market, and also some of the most expensive. There's some good articles in there, too.
Another great source for exhaust merge collectors, only a bit cheaper and with nearly infinite custom configurations.
A Carcraft article that explains what merge collectors do and why they are important.
Vendors / Other
Jack Money has knowledge and experience with the 325ix - something few online vendors have.
These guys have built some awesome ix rally cars.
A rally team where they use a couple 325ix's to compete in the Alcan 5000 winter rally.
Metric Mechanic builds some pretty stout M20 strokers (and other BMW engines of course). I don't like everything about their motors, but their customer support is fantastic and if you want to learn something about building an M20, give them a call and talk to Jim.