Tax revenue is the main source of government revenue, either direct taxes or indirect taxes.
The sale of goods and services can be a source of government revenue via state-owned enterprises such as a national postal service.
Privatization refers to selling state-owned enterprises and government-owned assets for government revenue. This is only a short-term policy as assets can only be sold once to the private sector.
Sovereign wealth funds are state-owned investment funds such as stocks which provide government revenue.
If there is not enough government revenue, the government can borrow from the public sector (Public sector borrowing).