FRSport are absolutely spot on. I've used them loads of times now! Overall probably the cheapest place to get JDM parts (even though they are in the US! Work that one out!) By farrr the place I use most and my most trusted/favourite out of the lot.
TheNismoShop (West Covina Nissan). I've used these before, similar to FRSport, great prices. Shipping took a while due to my card being rejected as my bank didn't allow the international transaction, took a while to get a reply when I contacted them which was worrying. Otherwise a superb service.
I've used Nengun before, good service although took forever! The prices aren't exactly cheap on there either!
RHDJapan are ok, very similar to Nengun really. Nothing is amazingly priced, good service though
Touge Distribution, friend has used these before - no problems at all!
Newera Parts, i've heard a lot of people slate this lot. I used them and I got a part over £100 cheaper than anywhere else I could find. If your after a specific part you can get anywhere else for a half decent price they are well worth a try. Mate bought £1600 worth of Work wheels from them too!
Apex Performance, sell some really handy stuff you can't get anywhere else. Shipping is always prompt although in all honesty they charge stupidly expensive prices. They don't give anything away either, only good value stuff is their own which isn't always what your after.
Driftworks, superb all day long. Not a bad word to say about Driftworks. Customer support is superb, items are always recieved next day (Unless you choose 1st Class Recorded Signed For - if you aren't in your postman takes it back to the local sorting office, which for me is major hassle!!!!)
Japspeed, actual service is ok. All their items are useless and overpriced. (Most of the ebay special stuff comes in Japspeed boxes! I once bought a Japspeed superlow downpipe for £47 incl postage from ebay before and it was a Japspeed one! Don't fool for it!) Wouldn't ever touch Japspeed again!
DemonTweeks, superb, stock loads of stuff, shipping is uber quick!! You pay the price though! This lot don't give stuff away, not really a great range of JDM stuff either! They do sell some tat as well!
DB-Power, good selection of brakes. Not the greatest place in the world but have some good stuff, especially on the SXOC.com group buys
Used loads of other places, these are the main ones. Whenever I look for something though, FRSport it is.. every time!