struts-<version>.jar file. The version could vary as that of the implementation. You may get struts-1.2.1 jar say for example if you download the 1.2.1 version.
You can do it by two ways.
Place the jar file inside your web applications "WEB-INF/lib" directory so that it will be useful ONLY to the specific web app.
Place it in the <TOMCAT_INSTALLATION_DIR/shared/lib" directory so that it will be useful for all web applications deployed in that instance of tomcat which uses Struts. - The common library files required by many applications will follow this way.