Most students write the same three exams. SSC CGL, UPSC CSE, and whatever their state PSC runs. Everything else in this list also pays a central government scale, and almost nobody checks it.
Two things to understand before the list. Some of these have a fixed annual exam you can plan a year around. Others recruit only when a vacancy opens, which means the whole game is watching the notification rather than preparing for a date. They are separated below for that reason.
Indian Statistical Service (ISS) Eligibility: Bachelor's with Statistics, Mathematical Statistics or Applied Statistics, or a Master's in Statistics. Exam: UPSC ISS. Timing: exam in June, notification around February. The officers who work with government data, surveys and the statistics behind policy.
Indian Economic Service (IES) Eligibility: Master's in Economics, Applied Economics, Business Economics or Econometrics. Exam: UPSC IES. Timing: exam in June, notification around February. Officers who advise the Government of India on economic policy and planning.
Central Secretariat Service (CSS) Eligibility: graduation. Route: mainly Assistant Section Officer through SSC CGL. Serving officers also have departmental promotion exams. Timing: SSC CGL runs around August to September, varying by year. Administrative officers who run central ministries from New Delhi. Note that the entry route here is SSC CGL, which is the most crowded exam in the country. The post is under-known; the exam is not.
Food Corporation of India (FCI), Manager Eligibility: graduation, with some posts requiring CA, MBA, Engineering, Agriculture or Law. Exam: FCI Category II Manager. Timing: no fixed annual schedule. Manages India's food procurement, storage and distribution.
Cotton Corporation of India (CCI) Eligibility: graduation, B.Com, MBA, Agriculture or Engineering depending on the post. Exam: CCI recruitment exam. Timing: released as vacancies arise. A PSU that procures cotton and supports the industry around it.
Airports Authority of India (AAI) Eligibility: graduation, Engineering or MBA depending on the post. Exam: AAI Junior Executive and Manager recruitment. Timing: usually one or two recruitments a year, not on a fixed date. Operates India's airports and air navigation services.
These four have no exam you can prepare for on a calendar. The application window is the opportunity, and missing the notification is the only way to lose.
National Testing Agency (NTA) Eligibility: depends on the post. Graduate, Engineer, MBA and others. Route: direct recruitment through NTA notifications. The body that conducts NEET, CUET and JEE Main.
National Highways Authority of India (NHAI) Eligibility: mostly Engineering, Law, MBA or graduation depending on the post. Route: direct recruitment, deputation, and GATE for some technical posts. Builds and manages national highways and expressways.
National Seeds Corporation (NSC) Eligibility: Agriculture, Botany, B.Sc., MBA and others depending on the post. Route: NSC recruitment notifications. A PSU producing and supplying certified seed.
Indian Renewable Energy Development Agency (IREDA) Eligibility: Engineering, Finance, Law, MBA or CA. Route: direct recruitment. Finances solar, wind and other clean energy projects.
Six of these have an exam. Four do not, and for those four the entire skill is noticing the notification in the week it opens.
That difference is worth naming, because it changes what preparation means. For ISS and IES you need a year. For IREDA or NSC you need to be watching.