Schema-less data in a database? I'm wondering if there's any database technology where its structure or schema does not need to be defined upfront? For example it may start off with just one column or row and the user will keep adding more columns and rows to the table on the fly, when they see it necessary.
If you write your queries properly and avoid SELECT * then any relational database will allow the addition of further columns or tables without requiring adjustment to the application.  You do, of course, have to declare each column to the DBMS before referencing it in SQL.

I find the claim that NoSQL is "schema-less" to be slightly misleading.  Applications using NoSQL persistance do, indeed, have a schema.  The difference is that the schema is held in application code and it is the responsibility of every programmer who touches the code throughout the application's life to enforce that schema.  With relational databases the data structure is declared to the service and it then takes the responsibility for enforcing those rules for ever after.  The real flexibility of NoSQL is that two rows within the same "bucket" (the definition of which various depending on your DBMS) can have different structure.