Base Metals: Copper, zinc, tin, boron, lithium, germanium, platinum, indium and to a much lesser extent these days, nickel.
In general, Sterling Silver is 92.5% silver, and 7.5% of "something else". The vast majority of the time that "something else" is copper. Copper is nickel free and has the right balance of color, durability and cost to make it the number one choice by raw sterling manufacturers. The formula for Sterling is always being refined to give it better properties.
Rhodium is sometimes used (a member of the platinum group) to plate sterling silver to give it the look of white gold. Unfortunately, rhodium does not stick easily to silver so another metal is often used to coat the silver and the rhodium is plated over that "glue" metal. What is a very popular metal for that "glue" process? You guessed it, nickel. The problem is that rhodium is fairly brittle and can form microscopic cracks. Now the underlying nickel can get moist from your skin and leech out nickel salts. All of a sudden you have an allergic reaction to sterling silver, which is really the underlying nickel causing the rash. It is also possible for the jewelry piece you have purchased, to be plated with nickel. Because Sterling silver tends to wear and tarnish over time some manufacturers plate their pieces with nickel to give it a long lasting shine.
Alpaca Silver, German Silver, Paktong and New Silver: These are actually different names for Nickel Silver, a silver type that, as the name suggests, will likely contain nickel. These types of silver should be avoided! In addition, when shopping, try to avoid older pieces that may have been produced when nickel plating was a more common practice.
Since there seems to be no economic benefit to using nickel, and since regulation limiting the use of nickel is spreading, you really should put sterling silver on your list of safe to wear items. The good news is that it is very unusual for sterling silver to contain nickel anymore, but it doesn't hurt to ask!
My best advice... Always shop where your "satisfaction is guaranteed or your money back"!