A lot of people still want to use the older twin-blade razors rather than switch to 3, 4, or 5-blade razors. (And the Korean company Dorco recently came out with a 6-blade razor. Where will the madness end?) It takes a bit of doing to figure out which razor will work with which blade. This guide will help you know which type of blade fits which razor.
Many people also stick with double edge and injector razors, but there's no mystery about which blades fit which razor. All common double edge blades are now made to fit virtually all double edge razors. Though there is actually more than one size of injector blade, the most common size (the only one you'll find in a drug store) is the one that fits the standard injector razor.
Below, I will almost always use the word "blade" to mean "cartridge" or "blade cartridge".I encourage you to send me corrections, updates, and additions.
Here are the razor types I know about:
Gillette Trac II, GII, PII - In some countries, this is called the GII or PII, but it's the same thing. (Personna also makes the same kind of razor with the name PII.) This type uses a slide-on blade, with no pivoting. Many other companies made compatible razors and blades. Almost any slide-on blade and razor you find is of the Trac II type. The word "Plus" added to the name simply refers to the extra feature of a lubrication strip added along the edge of the cartridge, right above the top blade. The German brand Merkur makes plastic adapters that slide onto a Trac II type razor, and allow it to use Atra or Schick Protector blades.
Other brands that fit - Personna Twin II, Personna PII (or PII Plus), Personna Duofit, Schick Super II, Schick Personal Touch, Bump Fighter (one blade with guard), HOK2 (Squeeze 'n' Shave, Wally, and Spike), Headblade, Merkur. Most store brands are made by American Safety Razor Co., which makes Personna. Feather, Merkur, Personna, and others make razors that use Trac II blades.
UPDATE - The Wally and Spike razors have been redesigned to use a pivoting triple blade. However, I don't know what kind of blade it is. (My best guess would be Sensor, but that's a guess. They sell their own brand, and don't tell you about other compatible brands.)
Gillette Atra, Contour, Vector - The blade clips onto the handle at two points and pivots up and down, which is supposed to make the blade follow the curves of your skin better than the Trac II and earlier razors. The Vector may be the only one still being made. It's almost identical to the later Atra and Contour that used a plastic handle rather than a metal one. I think the word "Plus" in the Vector means something different from other Gillette razors. Instead of referring to the lubrication strip, it refers to a push mechanism in the cartridge that cleans out the hair between the two blades.
Other brands that fit - Personna Auto, Personna Duofit, Schick Super II, Schick Slim Twin (the cartridge is thinner, but should still fit; this is not the current disposable Slim Twin razor), Schick Ultra Plus, Wilkinson Contact Plus, Wilkinson Profile, Old Spice High Endurance (triple blade), Noxzema Triple Blade, Headblade, Merkur. Ruud & Rye (of Norway) offers a razor that uses Atra blades. Ditto for Old Spice, but that razor is no longer sold in the USA.
Note that some blades are made to fit both Atra and Trac II type razors. On some pivoting razors, this kind of blade may not pivot as much as a blade without Trac II fittings.
UPDATE - Compatibility isn't exact with the Schick Slim Twin. I've had success using the Slim Twin cartridge in more than one pivoting razor, but other people have reported problems.
Gillette Sensor, Sensor Excel, Sensor 3 - Newer than the Atra, with somewhat different pivoting of the blade and the added feature of each blade in the cartridge flexing individually. The Sensor 3 uses a triple blade, but it fits the same. American Safety Razor also makes compatible handles and triple and twin blades.
Other brands that fit - Acti-Flexx (twin blade), Tri-Flexx (triple blade) - both of these will also be labeled "Personna" or have a store brand name.
Schick (or Wilkinson) Tracer, Tracer FX, FX Diamond, FX Performer - Some countries have the Wilkinson brand instead of Schick (both companies are now owned by Energizer), and also use the name "FX Performer". All of these razors use the same kind of blade, which also uses the same names as the razor. (There are differences between the blades with different names, but all of them fit all of these razors.) The blades attach at two points, and pivot slightly. The main feature is that the blade is flexible and bends inward to follow the curves of your skin.
Schick (or Wilkinson) Protector, Protector 3D [Diamond], Silk Effects - All of these have interchangeable blades, though there are some differences not related to fit. This type has a pivoting cartridge, with the blades wrapped in thin wires to protect against nicking the skin. The blades move within the cartridge to follow the curves of the skin. The interesting, less-known fact is that the women's Silk Effects razor is part of this group.