What will be the fastest way to implement a good search on my site?

Question

Say I have a site like stackoverflow. I need to add a search functionality, I need it to be good. I don't care if I have to add a logo (like if I am using a google search). What options do I have, with minimum of development and minimum server type dependency.
I am currently on Lamp (php).

EDIT:
Pardon, but I am talking about an open source application, which each user is installing on his server-machine (hence the requirement of not server specific). The goolge search, as far as I can see, I have to subscribe to it first, A process I wish users not have to do.

Answer 1

The Easy Way Out would be to add a custom Google search.

http://www.google.com/coop/cse/

Answer 2

Untested, but something like this has worked for me in the past:

<form action="http://www.google.com/search" onSubmit="this.q.value+=' site:mysite.com';return true">
<input name="q" />
</form>

Otherwise, perhaps this: http://www.google.com/sitesearch/

Answer 3

Ok so you added in the comment you're looking to integrate search into a desktop application. I've used ht://Dig with pretty good success in the past, but it seems to have lost some momentum. More advanced, modern applications tend to use Apache Lucene which will require a significant amount of integration.

If you're looking for something bit higher level, check out solr, which classifies itself as an "open source enterprise search server".