Since there are often a lot of questions about classing for SCCA Autocrossing, I figured I'd put a thread together. Since I've exclusively autoxed Mazdas, I can answer mostly to that (P5, MSP, Miata). I'll try and put as much in, and specify if there's a certain mod that caters more to 1 car than the rest (*cough*miata*cough*). Of course, these lists are not all-inclusive but give a pretty solid overview. As always consult the rulebook for specifics. The classes generally build on themselves, so if there's a mod listed in a lower category that's not listed in a higher one, chances are it's allowed.
Shocks, front sway, r-comps
Wheels up to 7.5" wide, street tires up to 140 treadwear, braided brake lines, shocks, springs, bushings, race seats, engine bolt-ons (cats have to remain), aftermarket brakes.
Update/Backdate Engine swaps, no-top (UDBD for Miatas), full ECU, ITBs, any wheels, fat r-comps, fender trimming, race seats, big rear spoilers, LSDs, UD/BD transmissions (Miatas), aftermarket brakes, flywheels & clutch
Any block from your car's manufacturer, engine mods are wide open, aftermarket (or modified stock) forced induction is encouraged, huge tires, huge wheels, big wings. Minimum weights based on displacement, body mods allowed
Must use stock block, no aftermarket FI, but other than that engine allowances are wide open. Seam welding is allowed. Serious weight reduction, suspension is fairly open including improving geometry, fuel cells. Open cars MUST run a rulebook legal roll bar (2" under, etc), cages are encouraged, cars will run R-Comps or full slicks.
Classes where nearly anything goes. Home built specials to formula cars to turbocharged rotary bugeyed sprites running Formula Continental tires. If you can think of it, it PROBABLY has a place to play in Mod. Rules vary widely for "production" based cars and formula cars, consult the rulebook for more info. But if you have a gutted out turbo'd car, it runs here.