If you have a recent version of Maven and you wanna do a release or build a jar without running all the unit tests (which you should run alot but now and again you might wanna skip) you can set
maven java:jar -Dmaven.test.skip=true
The same thing can be done if ever you're offline and want to stop Maven trying to download jars
maven test -Dmaven.mode.online=false
UpdateThanks to bob for spotting it. You can just use the -o or --offline option to work offline.
maven -o test