HipMojo lists the top 10:

#11 - Honorary Mention: Yahoo! acquires Yoyodyne in 1998 for $39 Million

#10 - aQuantive acquires Razorfish for $160M in June 2004

#9 - Microsoft acquires Hotmail for $400M in 1998

#8 - Ask Jeeves acquires Interactive Search Holdings for $343M in March 2004

#7 - eBay acquires Paypal for $1.5B in 2002

#6 - AOL Time Warner acquires Advertising.com for $435M in June 2004

#5 - Yahoo! acquires Inktomi for $235M in December, 2002

#4 - Yahoo! acquires Overture for $1.63B in July, 2003

#3 - Google acquires Sprinks (for basically nothing) in October 2003

#2 - New York Times acquires About.com for $410M in February, 2005

#1 - News Corporation acquires MySpace-parent Intermix for $580M in May 2005

Fred Wilson writes about the word's many meanings:

The first meaning, coined by Alvin Toffler in his prophetic book, The Third Wave, is the merger between the notions of producer and consumer. Toffler envisioned the world we now live in where the consumer is participating in the production of customized products and services.

But there is a second meaning, that of products targeted between the low end consumer market and the high end professional market. You hear it most often used in products in the audio, photography, and video markets.

And then there is the more activist meaning, which simply takes Toffler's meaning and extends it into a world where mass produced goods are bad and "do it yourself" is the mantra. We certainly have made a number of investments at Union Square Ventures which are based loosely on this theme, certainly Etsy and Bug Labs, but also to some extent delicious, FeedBurner, and Oddcast, play on the theme of consumers creating their own products and services.

Robert Cringely writes:

That success couldn't be simpler: FREE has a mystique that is hard to fight. That makes Craigslist Linux to eBay's Windows.
...
Craigslist will always win when it comes to local classified advertising, simply because Craig and Jim's operation is big enough and popular enough and cheap enough that nobody else will ever be able to catch up. Local classified advertising on the Internet is Craigslist's opportunity to lose, not any other company's opportunity to win, because it can't be done.