I'm not sure if it markes a coming of age for online advertising or signals the return of totally bananas year 2000-style deals. But there's no doubt it's a good time to be a company with online advertising technology, and up-for-sale. Check out these deals from the past month or so, culminating in Microsoft's biggest buy, ever, announced over the weekend:

April 13: Google announces it will buy ad-serving specialist DoubleClick for $US3.1 billion.

April 30: Yahoo buys Right Media, which operates a real-time online ad auction network, for $US680 million.

May 18: WPP Group, a bricks and mortar agency, agrees to acquire online ad firm 24/7 Real Media for $US649 million.

May 19: Microsoft announces $US6 billion deal to buy aQuantive, a Seattle-based holding company whose properties include A/Razerfish, one of the largest online ad agencies.

What does it all mean? A smaller and smaller number of companies controlling the ads that are placed beside search results, around videos, and elsewhere on websites, funding almost everything you see online. Microsoft, Yahoo and various ye olde time ad agencies are trying to out-Google Google, while Google itself only grows ever huger.