JSON API
Every Ethereum node provides access to a JSON API which allows you to export blocks, transactions and other Ethereum objects in JSON.
To export Ethereum data, you may rely on a number of public nodes listed at ethereumnodes.com: some of them are free of charge.
A public node's API is usually accessible via HTTP (RPC), but when you run your own node, you may efficiently fetch the data using geth IPC (via a unix socket). This way you avoid the network overhead.
But to fetch a data from your own node, you have to wait for it to fully sync which may take several days. So, a public node is a good quick start option.
Alternative: BigQuery public datasets
You may also rely on BigQuery public datasets with Ethereum data. This will free your hands from exporting the data and allow to directly query it.
Further reading
Here is a detailed article about exporting the full history of Ethereum into S3: “How to Export a Full History of Ethereum Blockchain to S3” (Medium). It includes all the options: public nodes, owned nodes, BigQuery datasets, and tips on the further Ethereum data processing.