In Japan itself, there are three variants officially mandated for various uses:
Railway Standard (鉄道掲示基準規程) [2], in which the rendering of syllabic n as m before b, m, p is used. All JR railways and other major railways use this type for station names.
Ministry of Foreign Affairs standard (外務省旅券規定) [3], which explicitly allows the use of "non-Hepburn romaji" (非ヘボン式ローマ字 hi-hebon-shiki rōmaji?) in personal names, notably for passports. In particular, rendering the syllabic n as m before b, m, p, and romanizing long o as any of oh, oo or ou (e.g. any of Satoh, Satoo or Satou for 佐藤) is permitted.
Ministry of Land, Infrastructure and Transport standard [4] in which the rendering of syllabic n as m before certain consonants is not used. This is used for road signs.
Details of these variants can be found below.