Ross Mayfield's started a clever thread--what are your *ideal* aka dream mergers?
Some from the wiki via Ross and Jeff Clavier:

• SixApart acquires FeedBurner
• Google picks up Kayak.com
• IBM acquires JotSpot, connects its back-end to Lotus Notes, and makes it its entry level collaboration platform
• Microsoft buys Expedia
• Google buys Encyclopædia Britannica and makes the content available for editing via Wikipedia.