Lucene offers powerful features through a simple API:

• over 20MB/minute on Pentium M 1.5GHz
  
• small RAM requirements -- only 1MB heap
• incremental indexing as fast as batch indexing
• index size roughly 20-30% the size of text indexed

• ranked searching -- best results returned first
• sorting by any field
• many powerful query types: phrase queries, wildcard queries, proximity queries, range queries and more
• fielded searching (e.g., title, author, contents)
• date-range searching
• multiple-index searching with merged results

• 100%-pure Java
• implementations in other programming languages available that are index-compatible