Can you please point to alternative data storage tools and give good reasons to use them instead of good-old relational databases? In my opinion, most applications rarely use the full power of SQL--it would be interesting to see how to build an SQL-free application.
Plain text files in a filesystem
XML or JSON files on disk
Spreadsheet / CSV file
Subversion (or similar disk based version control system)
Berkeley DB (Basically, a disk based hashtable)
Native language collections (stored in memory or serialised on disk)
Custom (hand-written) storage engine
I can't claim to know anything much about them, but you might also like to look into object database systems.