Warlord Games/Bolt Action have the biggest range and cover pretty much every theatre of the war. They've also just recently bought the range of BEF minis, which gives them a host of early war vehicles and figures too. And they do plastics too.
Artizan Designs have a good 28mm range and are excellent quality too: http://www.artizandesigns.com/catalogue.asp?sub_range=2
CompanyB have a great range of vehicles that are nicely compatible with Bolt Action stuff: http://homepage.mac.com/brentdietrich/c ... decals.htm
There are heaps of them out there in 28mm:
Crusader Miniatures - http://www.crusaderminiatures.com/index.php
- great minis.
Offensive Miniatures - http://www.offensiveminiatures.com/index.asp
- superb range of US paratroopers and German Fallschirmjaeger
Wargames Foundry - http://www.wargamesfoundry.com/main.asp
- expensive but very nice figures
Army Group North - http://www.agnminiatures.com/index.html
- vehicles mostly
West Wind Productions - http://www.westwindproductions.co.uk/
- 25mm, so a little small but they have a very large range of vehicles and figures.
Eureka Miniatures (http://eurekamin.com.au/index.php?cPath ... 59&sort=3a
) do a small but exquisite range of pulp action WWII figures, plus a bit of weird war stuff (which I helped design).
I have minis from all of these companies and they all give excellent service.
Word of warning: Don't go near Black Tree Designs. While they have a huge range of figures and some tempting prices, they will take your money and you'll probably never see the figures. Loads of people have had problems with them, myself included.