The maths-reviewing service zbMATH offers free access to mathematicians and institutions in many developing countries. The exact list of which countries are eligible has just been revised, and can be found here. If you are a mathematician in one of these countries and would like access, then further information on how to obtain access can be found here.
If you are a mathematician who works in a developing country that is not on the aforementioned list, you may still be able to negotiate a free trial period. You are encouraged to email the editor of zbMATH with your request: