Lucene offers powerful features through a simple API:

• over 95GB/hour on modern hardware
  
• 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
• many powerful query types: phrase queries, wildcard queries, proximity queries, range queries and more
• fielded searching (e.g., title, author, contents)
• date-range searching
• sorting by any field
• multiple-index searching with merged results
• allows simultaneous update and searching

• Available as Open Source software under the Apache License which lets you use Lucene in both commercial and Open Source programs
• 100%-pure Java
• Implementations in other programming languages available that are index-compatible