Plain text files in a filesystem
- Very simple to create and edit
- Easy for users to manipulate with simple tools (i.e. text editors, grep etc)
- Efficient storage of binary documents
XML or JSON files on disk
- As above, but with a bit more ability to validate the structure.
Spreadsheet / CSV file
- Very easy model for business users to understand
Subversion (or similar disk based version control system)
- Very good support for versioning of data
Berkeley DB (Basically, a disk based hashtable)
- Very simple conceptually (just un-typed key/value)
- Quite fast
- No administration overhead
- Supports transactions I believe
Amazon’s Simple DB
- Much like Berkeley DB I believe, but hosted
Google’s App Engine Datastore
- Hosted and highly scalable
- Per document key-value storage (i.e. flexible data model)
CouchDB
- Document focus
- Simple storage of semi-structured / document based data
Native language collections (stored in memory or serialised on disk)
- Very tight language integration
Custom (hand-written) storage engine
- Potentially very high performance in required uses cases
I can’t claim to know anything much about them, but you might also like to look into object database systems.