Run like:
$ mvn help:describe -DartifactId=maven-war-plugin -DgroupId=org.apache.maven.plugins
for plug-in that have no version. You get output:
Name: Maven WAR Plugin Description: Builds a Web Application Archive (WAR) file from the project output and its dependencies. Group Id: org.apache.maven.plugins Artifact Id: maven-war-plugin Version: 2.2 Goal Prefix: war
Use version that shown in output.
UPDATE If you want to select among list of versions, use http://search.maven.org/ or http://mvnrepository.com/ Note that your favorite Java IDE must have Maven package search dialog. Just check docs.
SUPER UPDATE I also use:
$ mvn dependency:tree
$ mvn dependency:list
$ mvn dependency:resolve
$ mvn dependency:resolve-plugins # <-- THIS
Recently I discover how to get latest version for plug-in (or library) so no longer needs for googling or visiting Maven Central:
$ mvn versions:display-dependency-updates
$ mvn versions:display-plugin-updates # <-- THIS