The Legal Information Institute (LII) is a non-profit, public service of Cornell Law School that provides free access to legal research sources online. Its collection includes not only the USC, but federal court decisions and regulations, a uniform and model code locater, topical libraries, and Wex, a free legal dictionary/encyclopedia. The discussion here focuses on LII's version of the USC.
There are multiple places to find the USC online for free, whether from government, non-profit, or commercial sites. LII is worth noting in particular as it is one of the best designed and is the work of a respected non-profit.
When turning to statutes after considering secondary sources, it can be cost-effective to start with a free, easy to use resource like LII. It can help you get a sense of what statutes are on point and familiarize yourself with the USC's organization. After searching LII's USC you should turn to the annotated codes, USCS and USCA, whether in print or online, for more thorough research.
There are four primary ways to search LII's USC:
There's limited terms and connectors searching (* acts as wildcard, phrases in "double quotes"); unfortunately, there's no index. If you know the popular name of a law as it was passed, you can look the law up to see where it was codified in the USC. (LII provides a good explanation of what a "popular name" is.) LII's popular name table goes further than others in that it also provides CFR citations that derive from the statute.
Once you have located a particular section or title, the following tabs normally appear:
LII also offers a couple of links in the upper right that are worth highlighting:
Overall, LII does a good job of integrating and reworking government sites related to federal legislation into a more seamless and user-friendly resource. One advantage of the LII over other online free sources of the USC is the Authorities (CFR) feature.
The main disadvantage of the LII is that it is unannotated, and thus does not include citations to related secondary authority or case law involving a statute. Moreover, it has no citator equivalent to Shepard's or KeyCite, and does not allow you to see whether the statute is still good law. For instance, a statute could be declared unconstitutional, but it remains in the Code until Congress repeals or amends it. So, before relying on any statute found through the LII, it is important to use annotated versions of the code, such as the USCS or USCA, and Shepard's or KeyCite to check if the statute is still good law.
While the LII does include a Table of Popular Names, it does not include an index or other various tables found in any print version of the USC. (The omitted tables cover executive orders, proclamations, reorganization plans, lists of revised sections, the Statutes at Large, and internal cross-references within the Code. The subject index & tables can be useful when keyword searches are not fruitful.)
Congress is in the process of rearranging the Code and adding new titles, and LII might be slower to reflect these changes than the USCS and USCA. (LII had a bit of a lag in featuring the newest Title 51.)