OK, hope this is of use to some of you. I know I'm missing loads, especially in the wheels and bodykits. But, there are literally HUNDREDS of wheel manufacturers in Japan, and many more companies doing bodykits. I'll add to this as and when I find 'em. Or feel free to add to them.
http://world.altavista.com - Babelfish's translator. You may need this to actually read some of the following. Happily some of them have English info anyway. Enjoy
Note I have tried to group them into categories they are best known for, but there is inevitably some crossover - e.g Trust make aero kits, Veilside make tuning parts.
General Tuning Parts
5 Zigen - http://www.5zigen.co.jp
~ Exhaust specialist. Also make wheels and other miscellaneous goods
Apexi - http://www.apexi.co.jp
~ Large company, wide range of engine, suspension, brake, turbo and electronic components.
Arc - http://www.arcinter.co.jp
~ Specialise in intercoolers, custom airboxes and other alloy fabrications.
Bellof - http://www.bellof.co.jp
~ HID conversions for cars not so equipped..
Blitz - http://www.blitz.co.jp
~ Major company, wide range of tuning components, also wheels and body kits.
Border - http://www.border-racing.com
~ Various tuning parts and aerokits.
Bridgestone/Potenza Suspension - www.bs-sus.ne.jp
~ Springs, coilover kits, looks quite cool.
Buddy Club - http://www.first-inc.co.jp
~ Select range of race orientated tuning parts. Also wheels.
C's - http://www.cs-comrade.co.jp
~ The best short shifters, a few other bits and pieces
Cusco - http://www.cusco.co.jp
~ Drivetrain and suspension component specialists.
Defi - http://www.nippon-seiki.co.jp/defi/ or http://www.defi-shop.com
~ Excellent gauges, electronic gadgets, HUD's.
Endless - http://www.endless-sport.co.jp
~ Brake specialist. Zeal are also on this site - they make bodykits, suspension etc.
Field - http://www.field-net.co.jp
~ Electronic tuning parts.
Fujitsubo - http://www.fujitsubo.co.jp
~ Major exhaust manufacturer. Also more modern "Power Getter" range at www.fgk.com
GAB - http://www.yrc.co.jp/yfc/gab/
~ Suspension manufacturer
GP Sports - http://www.gpsports.co.jp
~ Silvia/180SX parts and aerokits.
HKS - http://www.hks-power.co.jp
~ You know about these!!
Ichikoh - http://www.ichikoh.com
~ Major OE lighting maker. Also make HID conversion kits.
Ikeya Formula - http://www.ikeya-f.co.jp
~ Sequential gearchange conversions
Impul - www.impul.co.jp
~ A few parts for 200SX
JIC Magic - http://www.jic-magic.co.jp
~ Nice range of coilovers and some exhausts.
Jun - http://www.junauto.co.jp
~ THE Engine experts. Also make bodykits.
Kakimoto - http://www.kakimotoracing.co.jp
~ Exhaust manufacturers. Also major tuners and have nice selection of cars on the site.
Kazama - http://www.kazamaauto.co.jp/
~ Turbo kits, suspension, aero kits etc
Kei Office - http://www.kei-office.co.jp/
~ Mainly coilovers
KSP Engineering/Attain - http://www.ksp-eng.co.jp
~ Various tuning parts and accessories
Kg/mm - http://www.kgm.co.jp
~ Suspension
KYB - http://www.kyb.co.jp
~ AKA Kayaba. OEM and aftermarket shocks.
Mines - http://www.mines-wave.com
~ "High end" tuners. ECU specialist.
Monster Sport - http://www.monster-sport.com
~ Engine, suspension, exhaust etc. Race orientated
Nismo - http://www.nismo.co.jp
~ Nissan Motorsports
Ogura Clutch - http://www.oguraclutch.co.jp
~ Clutches!!!
Ohlins - http://www.carrozzeriajapan.co.jp/ohlins/ohlins.htm
~ Some of the finest suspension available
Omori Gauges - http://www.omorimeter.co.jp
~ Nice range of gauges
Power Enterprise - www.power-enterprise.co.jp
~ Engine parts, turbo upgrades etc.
Project Mu - http://www.project-mu.co.jp
~ Brake specialist.
Prof - http://www3.ocn.ne.jp/~sheepdog/
~ AKA "Sheepdog" exhausts and BOV's
Raybrig - http://www.raybrig.gr.jp
~ Lighting, nice range of fog/driving lights and "HID style" bulbs.
RS*R - http://www.rs-r.co.jp
~ Exhaust and suspension manufacturer.
Sard - http://www.sard.co.jp
~ Large range of engine tuning components. Also adapter kits for fitting Stack data-logging dash systems into your 200! Not all on this site unfortunately.
Tanabe - http://www.rd-tanabe.com
~ Excellent range of exhausts. Also some suspension kits.
Tein - http://www.tein.co.jp
~ Famous suspension manufacturer
Toda Racing - http://www.toda-racing.co.jp
~ High quality uprated engine internals, and cheaper than Jun, Tomei, HKS etc.. US site at www.todaracing.com
Tokico - http://www.tokico.co.jp
~ Manufacturers of fine shock absorbers
Tomei - http://www.tomei-p.co.jp
~ Wide range of engine parts.
Top Fuel - http://www.topfuel.to
~ Some parts for Nissans. Most famous for high powered Hondas
Top Secret - http://www.topsecretjpn.com
~ Select range of tuning parts. Most famous for their monster Supra's and Skylines.
Trust - http://www.trust-power.com
~ Major tuning firm. All manner of engine, suspension and electronic goodies. Also manufacture bodykits.
Ultra - http://www.mmjp.or.jp/ULTRA/
~ Mainly electronic goodies
Whiteline - www.whiteline.com.au
~ Not Japanese, Australian but top quality and value footwork parts
Zees - http://www.zees.co.jp
~ Some rather "different" looking exhaust systems!
Wheels
Advan - http://www.advan.com
~ Some of the best wheels going!
Crimson - http://www.crimson-inc.co.jp
~ Makers of various wheels including Racing SPARCO, Myrtle and Hokuto brands
Enkei - http://www.enkei.co.jp
~ More wheels!
KYH - http://www.kyh.co.jp
~ Makers of wheels like the well known Circular GTA
Manaray Sport - http://www.marukanet.co.jp/
~ Some nice designs, some of them at surprisingly good prices!
Racing Hart - http://www.hartwheels.co.jp
~ Nice wheels, sold under Lowenhart, Sprint Hart and Racing Hart brands.
Rays Engineering - http://www.rayswheels.co.jp
~ Makers of some of the best known Japanese wheels, including Volk Racing, Versus and Nismo.
RE Amemiya - http://www.re-amemiya.co.jp
~ Mainly involved in tuning RX7's but also make some wheels...
SSR Wheels - http://www.speedstar.co.jp/
~ Nice range of rims
Superstar Wheels - http://www.superstar-wheel.com
~ Nice wheels. Nicer company slogans! Brands include Leon Hardiritt, Pure Spirits, Lodio Drive. Note - These are available from Elite wheels in the UK (at a price!!)
Watanabe - www.rs-watanabe.co.jp
~ Wheels. Favourites for drifting.
Weds - www.weds.co.jp
~ Best known for wheels. Some other stuff too.
Work - http://www.work-wheels.co.jp
~ Quality wheels, including the much copied Work Meister - at a price.