I'm currently playing with the awesome Apache Solr, and preparing to install it on my production server. Here are a few links I've found that I believe have good value.

First, there's the code you'll need to make it work:

• http://lucene.apache.org/solr/#getstarted <-- click the Download Solr "here" link
• http://drupal.org/node/605712 <-- Apache Solr Drupal Module

The a few tutorials on setting things up:

Installing Java 1.6 on MediaTemple dedicated virtual server (dv)
http://www.jaysonjc.com/2008/01/13/installing-java-16-jdk60-on-mediatemp...

http://drupal.org/node/576008 <-- Apache Solr and cron on Media Temple dv

Setting up Apache Solr
http://lucene.apache.org/solr/tutorial.html

Setting up the Apache Solr Drupal Module
http://drupalcode.org/viewvc/drupal/contributions/modules/apachesolr/REA...

Some more help guides:
http://drupal.org/node/343467 <-- Apache Solr Search Integration

Customising things:
http://drupal.org/node/598970 <-- Apache Solr "OR" Syntax
http://drupal.org/node/230412 <-- Real hierarchical taxonomy in the index
http://drupal.org/node/401234 <-- Reflect hierarchical taxonomy vocabulary in facet
http://drupal.org/node/504804 <-- Making Apache Solr the only search engine available
http://www.brightlemon.com/knowledge-base/blog/customizing-apache-solr-s... <-- Customising search results
http://drupal.org/node/492250 <-- More on customising search results (touches on theming)

Setting Apache Solr up as a service so it automatically starts
http://drupal.org/node/507292 <-- On Ubunutu

Drupal Apache Solr Search Multi Site Setup
http://www.anovasolutions.com/content/drupal-apache-solr-search-multi-si...

Dont' have a dedicated server? No worries, Acquia sell a hosted Apache Solr Solution!
http://acquia.com/products-services/acquia-search

And out of left field, a few interesting Drupal bits:
http://drupal.org/node/492494 <-- Shared Email module so more than one user can have the same email address
http://drupal.org/node/201594 <-- Programmatically creating nodes with ImageFields