PostgreSQL

There are various search techniques for text:

• Full text search
• Trigram search

Other useful search-based modules:

• unaccent removes accented characters for building out the search dictionary
• fuzzystrmatch implements Double Metaphone search support

There are Ruby on Rails gems that can take advantage of these (like pg_search)

Interesting reading about Postgres:

• Nine ways to shoot yourself in the foot with PostgreSQL, by Phil Booth