I can install an artifact by install:install-file,
but how can I download an artifact?
For example:
mvn download:download-file -Dgroup
With the latest version (2.8) of the Maven Dependency Plugin, downloading an artifact from the Maven Central Repository is as simple as:
mvn org.apache.maven.plugins:maven-dependency-plugin:2.8:get -Dartifact=groupId:artifactId:version[:packaging[:classifier]]
where groupId:artifactId:version, etc. are the Maven coordinates
An example, tested with Maven 2.0.9, Maven 2.2.1, and Maven 3.0.4:
mvn org.apache.maven.plugins:maven-dependency-plugin:2.8:get -Dartifact=org.hibernate:hibernate-entitymanager:3.4.0.GA:jar:sources
(Thanks to Pascal Thivent for providing his wonderful answer in the first place. I am adding another answer, because it wouldn't fit in a comment and it would be too extensive for an edit.)