AjaxSearch obviously ...
AjaxSearch for me concentrate lot of IT interesting subjects:
- build the right mySql request, which fit exactly the request user (results/performances)
- algorithm to provide the extracts where a search term has been found
- ajax (with mootools library) and js to display the results
- manage utf8 but also non multibyte charset, to do for instance a search with accented characters
- input form and security issues (like XSS)
- php (which needs debugging and profiling)
and soon jquery and xpdo ...
And the search feature is the key feature on internet. Look at Google ...
But may be I am not impartial