In general, the 300 series of stainless is non-magnetic except when extremely worked and is non-hardenable. 303 is easily machined but the least corrosion resistant of the 300's. 304 and 316 are in ascending order of corrosion resistance. The "L" grades of 304 and 316 are recommended for welded applications because they are lower in carbon, and less subject to weld cracking. 321 and 347 are also very corrosion resistant and resist embrittlement when they are welded. They are relatively difficult to machine.
The 400 series are magnetic and hardenable because of carbon content. Knives are often made of magnetic stainless. 416 stainless is less corrosion resistant than 303 and has about the same machining characteristics. It is rarely desirable unless parts need to be magnetic. 440C is capable of greater hardness and is more difficult to machine. 17-4PH is a precipitation hardening stainless which can be hardened to 40-45 Rockwell without distortion because of the low hardening temperature required.
Hastelloy is more corrosion resistant than stainless and more difficult to machine. The same is true of Inconel.
Kovar is a metal whose coefficient of thermal expansion closely matches that of some glass, so it is used for glass to metal seals or in places requiring low thermal expansion. It can be a fairly difficult material to machine.
Nickle 200 is almost pure nickel and is stringy and abrasive. A material used in corrosion resistant applications that is difficult to machine.
Monel, an alloy of Nickel and Copper, is also a high corrosion resistance material (used in the legs of Texas Towers). It also resists cutting tools.
Titanium is more widely used now, and is a very lightweight, somewhat difficult material to machine. It is also desirable for its very good chemical resistance, even being used within the human body for some medical products.
Molybdenum is a very high temperature resisting element, sometimes used in gas-shielded crystal growing furnaces as the seed holding chuck or as a structural member in vacuum tubes. It is generally difficult to machine.
