After Microsoft bought Lookout, by far the best Outlook indexer, they incorporated its functionality into Windows Desktop Search. WDS is a horrible app, in my opinion. Microsoft chose to intentionally disallow Lookout as an add-in in Outlook 2007. I have tried other indexers that work with Outlook, but none could compare to the efficient and fast Lookout. I settled on Nelson Email Organizer (NEO), which is a standalone application. It does do some nice things, but it just isn’t the same.
A successor to Lookout has found: Lookeen. Like Lookout, it is a COM add-in just for indexing your mailbox (not a bloated app that also indexes your mailbox). It is very fast, and offers a nice feature that Lookout never did: it display results in a tabbed window, so you can view results by their item type (messages, appointments, contacts, files, etc.). Lookeen is in beta right now, but you can download it and give it a try. I haven’t tried any complex searches, but my early results are very positive.