I can only really talk about the "traditional" companies that have been around a long time, i.e., Airfix, Fujimi, ESCI (now apparently being re-released by Italeri?), Hasegawa and Matchbox (now being produced by Revell Germany along with their own line of vehicles).

Airfix is 1/76 and has always been so. Although some of their kits are old and lacking in detail (after all, many of their kits were mastered in the early 60s - have a look at some of the catalogs at this Airfix figure site: http://soli.inav.net/~edzwil/), one thing that has been relatively good is their scaling. Airfix was originally marketed as "HO-OO" which was supposed to be a midway scale between 1/87 (HO) and OO, I believe and is 1/76. In 1994, on the 50th anniversary of D-Day, the owners of Airfix at the time (Airfix changed hands several times over the years) re-released a lot of great old Airfix kits but curiously enough advertised them as 1/72 scale.Fujimi and Matchbox kits are also 1/76.

ESCI and Hasegawa are 1/72 kits. ESC is relatively consistent, but the Hasegawa scaling varies wildly. The classic example is their 76mm Sherman HVSS. Like all Hasegawa kits, this is very nice (though basic by today's standards, having been released in the 70s) but huge. It is much bigger than the 1/72 ESCI (beautiful) Sherman kits and positively dwarfs the Airfix Sherman. I have several ESCI shermans but don't use them because of the size discrepancy. On the other hand, ESCI Shermans are a bit bigger than the Airfix ones, but for wargame purposes, Airfix and ESCI can be mixed OK.

Back to Hasegawa, some of their other kits match Airfix and other 1/76 scale kits better. The Hasegawa Crusader III, for example, is an exact match in size to the Airfix offering. Anyway, there's loads of comparisons to be made, so if you have any in mind, just post.

My experience with the new line of Revell kits is limited having been totally turned off by their abysmal Stug IV and Sherman 76 kits. Though a recent construction of their Panther G makes me realize those fellows who are raving about the quality of kits that came after the Sherman/Stug IV are right! I have some experience with Skif, Eastern Express/Cooperativa, PST, but others may have more.