Warning: visa requirements change rapidly. Please contact your local Japanese embassy or consulate to found out the latest requirements before embarking on your trip.
There are currently 59 countries that that no longer require visas for a limited stay. The full list can be found here: http://www.mofa.go.jp/j_info/visit/visa/02.html. For all other countries, you must get a visa from your local Japanese consulate prior to visiting Japan. Please note that period of stay varies between countries and work of any kind is not allowed.
If you are interested in working in Japan you must obtain a working visa. Serious penalties apply for people found working on a tourist visa. Working visas are available for the following professions: professor, artist, religious minister, journalist, business manager, legal, accounting, medical, researcher, instructor, engineer, humanities specialist, entertainer and skill labor. One must have at least a degree from a recognized university in their profession or at least 10 years documented, relevant work experience. Working visas are usually valid for one year and sometimes three years. While it is not legal to search for a job on a tourist visa, it is possible to apply for a working visa after arriving in Japan on a tourist visa without having to leave the country.
If you intend to leave Japan for a short period while on a working visa you must obtain a re-entry permit from immigration. Single and multiple entry visas are available.
You can find further information here: The Ministry of Foreign Affairs Web Site