I've finished up my search improvements patch for Drupal. It is now ready to be committed to core (if approved).

Changes:

• Clean up the text analyser: make it handle UTF-8 and all sorts of characters. The word splitter now does intelligent splitting into words and supports all Unicode characters. It has smart handling of acronyms, URLs, dates, ...
• It now indexes the filtered output, which means it can take advantage of HTML tags. Meaningful tags (headers, strong, em, ...) are analysed and used to boost certain words scores. This has the side-effect of allowing the indexing of PHP nodes.
• Link analyser for node links. The HTML analyser also checks for links. If they point to a node on the current site (handles path aliases) then the link's words are counted as part of the target node. This helps bring out commonly linked FAQs and answers to the top of the results.
• Index comments along with the node. This means that the search can make a difference between a single node/comment about 'X' and a whole thread about 'X'. It also makes the search results much shorter and more relevant (before this patch, comments were even shown first).
• We now keep track of total counts as well as a per item count for a word. This allows us to divide the word score by the total before adding up the scores for different words, and automatically makes noisewords have less influence than rare words. This dramatically improves the relevancy of multiword searches. This also makes the disadvantage of now using OR searching instead of AND searching less problematic.
• Includes support for text preprocessors through a hook. This is required to index Chinese and Japanese, because these languages do not use spaces between words. An external utility can be used to split these into words through a simple wrapper module. Other uses could be spell checking (although it would have no UI).
• Indexing is now regulated: only a certain amount of items will be indexed per cron run. This prevents PHP from running out of memory or timing out. This also makes the reindexing required for this patch automatic. I also added an index coverage estimate to the search admin screen.
• Code cleanup! Moved all the search stuff from common.inc into search.module, rewired some hooks and simplified the functions used. The search form and results now also use valid XHTML and form_ functions. The search admin was moved from search/configure to admin/search for consistency.
• Improved search output: we also show much more info per item: date, author, node type, amount of comments and a cool dynamic excerpt à la Google. The search form is now much more simpler and the help is only displayed as tips when no search results are found.
• By moving all search logic to SQL, I was able to add a pager to the search results. This improves usability and performance dramatically.