Here are a few links:
Forums and General
Leverguns.com Lots of good articles and resources as well as a forum with plenty of friendly, knowledgeable folk.
Marlinowners.com THE forum for Marlin enthusiasts.
Marlin Firearms Collectors Association
Single Action Shooting Society - cowboy action shooting at its finest.
Shooters Forum Forum sponsored by Beartooth Bullets- has a good levergun section
Sixguns.com The natural choice of sidearm for a levergun.
Winchester Arms Colectors Association
Gunblast.com Lots of gun reviews, some of which are leverguns. Be sure to check the archive.
Ballistics by the Inch What performance can you expect from pistol cartridges in a carbine? Includes .357, .41, and .44 magnums, .38 and .44 specials, and .45 Colt.
Levergun Manufacturers
Browning Makers of the BLR
Henry Repeating Side eject rifles from .22 to .30-30
Marlin Firearms Co. 'nuff said
O.F. Mossberg & Sons Model 464
Rossi USA Model 1892 rifles in .357 magnum, .44-40, .44 magnum, .45 Colt, and .454 Casull.
Winchester Repeating Arms Still selling limited edition 86, 92, & 95 rifles.
Ammunition Manufacturers
Buffalo Bore "Strictly Big Bore - Strictly Business" Hot ammo for many cartridges, even some that aren't big bore.
Cor-Bon Specialty ammo maker, lots of SAAMI spec ammo but with hot ammo for .45-70 and .45 Colt.
Double Tap Ammo Of special interest: Standard pressure loads for .44 Special, .45 Colt,and .45-70 that produce maximum performance without hotrodding.
Garrett Cartridge Considered the gold standard for magnumized .45-70.
Grizzly Cartridge A broad spectrum of hot ammo for leverguns and revolvers.
Firearm Importers and Distributors
Cimarron Firearms
Legacy Sports
Navy Arms Co.
Taylor's & Co.
Retailers
Buffalo Arms Company Outfitters to the old west. They have over 7,000 items for black powder cartridge and cowboy action shooting.
Dixie Gun Works Inc.
Restorations and Custom
Doug Turnbull Restoration Inc.
Reloading
Hornady Leverevolution data