How can I download a specific Maven artifact in one command line?

You could use the maven dependency plugin which has a nice dependency:get goal since version 2.1. No need for a pom, everything happens on the command line.

To make sure to find the dependency:get goal, you need to explicitly tell maven to use the version 2.1, i.e. you need to use the fully qualified name of the plugin, including the version:

mvn org.apache.maven.plugins:maven-dependency-plugin:2.1:get \
    -DrepoUrl=url \
    -Dartifact=groupId:artifactId:version

UPDATE: With older versions of Maven (prior to 2.1), it is possible to run dependency:get normally (without using the fully qualified name and version) by forcing your copy of maven to use a given version of a plugin.

This can be done as follows:

1. Add the following line within the <settings> element of your ~/.m2/settings.xml file:

<usePluginRegistry>true</usePluginRegistry>

2. Add the file ~/.m2/plugin-registry.xml with the following contents:

<?xml version="1.0" encoding="UTF-8"?>
<pluginRegistry xsi:schemaLocation="http://maven.apache.org/PLUGIN_REGISTRY/1.0.0 http://maven.apache.org/xsd/plugin-registry-1.0.0.xsd"
xmlns="http://maven.apache.org/PLUGIN_REGISTRY/1.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-dependency-plugin</artifactId>
      <useVersion>2.1</useVersion>
      <rejectedVersions/>
    </plugin>
  </plugins>
</pluginRegistry>

But this doesn't seem to work anymore with maven 2.1/2.2. Actually, according to the Introduction to the Plugin Registry, features of the plugin-registry.xml have been redesigned (for portability) and the plugin registry is currently in a semi-dormant state within Maven 2. So I think we have to use the long name for now (when using the plugin without a pom, which is the idea behind dependency:get).

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.)

Here's what worked for me to download the latest version of an artifact called "component.jar" with Maven 3.1.1 in the end (other suggestions did not, mostly due to maven version changes I believe)

This actually downloads the file and copies it into the local working directory

From bash:

mvn dependency:get \
    -DrepoUrl=http://.../ \
        -Dartifact=com.foo.something:component:LATEST:jar \
        -Dtransitive=false \
        -Ddest=component.jar \