A search engine module based on Sphider for Zen Cart Module was recently developed
(see

http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_47&products_id=1194).

This module is good for a web site with lot of EasyPages. But for a web
site with 50,000 products and structured data, Sphinx is still the best.

Features of Sphinx
SQL Full-text Search Engine

• Indexes via SphinxSE direct connection to
  MySQL or PostgreSQL
• Accepts data in XML format from using the Sphinx API
• Note that this package does not include
  a robot spider for traversing web sites.
• High-speed indexing, up to 10 MB/sec on
  fast CPU
• Can store up to 100 GB text / 100 million documents on a
  single CPU
• Indexes any number of fields and metadata
  tags, can be changed on the fly
• Stemming for English and Russian words
• Simple search modes, “match all”, “match
  any”, “match phrase”
  
• Also supports Boolean expressions (using
  &, |, -, and parentheses)
• Field searching, wildcarding, and proximity operators
• Fast searching (under 0.1 second on 2-4 GB
  index)
• Stemming query processing English and Russian natively, and
  with libstemmer
  for French, Spanish, Portuguese, Italian, German, Dutch, Swedish,
  Norwegian, Danish, Finnish, Hungarian
• Relevance based on BM25,
  (a flavor of TF:IDF) and phrase-based ranking.
• Sort by relevance, metadata in ascending or descending
  order, time range, or a SQL-like expression
• Group results by day, week, month, year, or an attribute
  value, and show only the best match per group.
• Can adjust relevance according to fields
  (even at runtime)
• Options for faceted metadata display
• XML results interface for custom
  integration
• Can replace SQL WHERE, ORDER BY, and GROUP BY query elements
• APIs for PHP, Python, Java, Perl,
  and Ruby
• Can utilize many CPU cores
• Distributed search – multiple search
  engines can search segmented indexes on multiple machines.
• Scales to at least 1.2 billion records, 30 million searches
  per day on a 15-server installation
• Source code and binaries available
• Administration by config file

Sphider Features:


Spidering and
indexing

    * Performs full text indexing.
    * Can index both static and dynamic
pages.
    * Finds links in href, frame, area and
meta tags, and can also follow links given in javascript as strings via
window.location and window.open.
    * Respects robots.txt protocol, and
nofollow and noindex tags.
    * Follows server side redirections.
    * Allows spidering to be limited by
depth (ie maximum number of clicks from the starting page), by
(sub)domain or by directory.
    * Allows spidering only the urls
matching (or not matching) certain keywords or regular expressions.
    * Supports indexing of pdf and doc files
(using external binaries for file conversion).
    * Allows resuming paused spidering.
    * Possbility to exclude common words
from being indexed.

Searching

    * Supports AND, OR and phrase searches
    * Supports excluding words (by putting a
‘-’ in front of a word, any page including the word will be omitted
from the results).
    * Option to add and group sites into
categories
    * Possibility to limit searching to a
given category and its subcategories.
    * Possibility of searcing in a specified
domain only.
    * “Did you mean” search suggestion on
mistyped queries.
    * Context-sensitive auto-completion on
search terms (a la Google Suggest)
    * Word stemming for english (searching
for “run” finds “running”, “runs” etc).

Comparison of Sphider and Sphinx PHP search engines

Ref: 
PHP Search Engine
Showdown http://www.onlamp.com/pub/a/php/2006/02/16/search-engine-showdown.html?page=2