The Ninth and Tenth amendments are something of an inseparable pair. The Ninth in particular has been almost entirely subsumed into the Tenth, and almost never gets mentioned in any binding opinion/decision. The Lost Jurisprudence of the Ninth Amendment by Kurt Lash is a fairly recent treatment of the Ninth amendment's jurisprudence (insofar as mostly historical accounts, dissents, and non-binding opinions count as such), and so details a lot about the Tenth amendment's jurisprudence as a result. It's a good read, provided one can handle law review style articles.
The short of the matter is that the 9th and 10th amendments, in the modern era, were basically gutted in their practical import by the New Deal era SCOTUS. Roosevelt's New Deal needed a vast expansion of federal power and authority to enact, and while initially it received a number of defeats at SCOTUS, by 1937 the (judicial) tide turned and many of these expansions were granted/recognized.
The power that Congress is exercising on these matters is the Commerce Clause. The power of this clause has waxed and waned over time, with a vast expansion of its potency beginning in 1937. Two prominent such cases are Wickard v. Filburn (1942; Congress can regulate goods that impact interstate commerce in the abstract, even if they are never actually sold or used across state lines, or even leave the possession of their producer) and West Coast Hotel Co. v. Parrish (1937). The latter case is what enabled minimum wage laws to be enabled, now justified by the Commerce Clause; this despite the fact that the court had ruled before that such laws were unconstitutional. The court then enabled laws prohibiting child labor, mandated overtime, etc. These were, again, all things the court had previously struck down as beyond Federal power, but now enabled and firmly set within that power. Federal drug laws all find their constitutional validity in the commerce clause, as well.
1937 was the transition point from SCOTUS-as-protector-of-economic-rights to SCOTUS-as-protector-of-civil-rights. The court heavily deferred to the judgement of Congress on the invocation of the Commerce Clause, holding it as a political question. It wasn't until United States v. Lopez in 1995, nearly 60 years later, that SCOTUS issued a ruling that actually struck down a law which was enacted based on the Commerce Clause. That law dealt with firearms near school areas, but was struck down as being too tenuous a connection to Commerce (basically, "schools are good for commerce, crime is bad for schools, guns abet crime, ergo guns near schools are bad for commerce and can be regulated on that basis" was held to be a stretch—even with the gun bits removed). However, Congress amended that law in response so that it only applied to guns that had crossed state lines, and this has been upheld to date as it specifically invokes the conditions of the Commerce Clause.
The Rehnquist Court restrained the clause's power in other cases as well. But nevertheless, in the modern era, where nearly everything has become commercialized on a multinational scale and nearly every commercial enterprise is thereby unavoidably and substantively tied to interstate (and international) trade, the Commerce Clause is extremely potent. The Necessary and Proper clause is also another source of potentially broad federal powers, but I won't get into that here.
For modern uses of the 10th amendment, you can see James' answer. In short, the 10th has been used to prevent the federal government from forcing state governments to enforce federal laws or pass certain laws. They can be encouraged to pass certain laws via federal funding requirements, but it can't be a coercive or mandatory thing (or be something unrelated to the subject of the funding).