I know this question has been asked countless times, but not until I started to mess around a little with this myself, did I discover that things to not go exactly as I hoped.
I have tried a few plugins, some just don't install, other need a page to display results other don't seem to do what they promise. It can actually be quite simple. Here is a piece code that does the trick simply by editing the search.php. I am no coder, so I can't get it to work the way I want. The displayed results look funky. The excerpt of the posts get the size of my metadata (really small). Also I would prefer if the search results could page. Last, but not least, an option to search sitewide or just the current blog would be nice as well.
Can the above be accomplished with a little editing of the code of Lars or does anybody know of a plugin that does do what I'd like?
 Oh, a more serious problem with the code is that when you search for something that has no result in the site where you're at, it doesn't continue to search the other sites!