William Slawski has compiled two very useful lists of acquisitions, first by Google and now by Yahoo (see also The Guardian's list). I'll add the third, Microsoft's acquisitions as listed on their corporate site. From 2003 on:

Nov. 3, 2005: FolderShare 
Nov. 3, 2005: media-streams.com AG 
Aug. 30, 2005: Teleo Inc. 
Jul. 20, 2005: FrontBridge Technologies 
Mar. 10, 2005: Groove Networks Inc. 
Feb. 8, 2005: Sybari Software Inc. 
December 16, 2004: GIANT Company Software 
April 26, 2004: ActiveViews 
April 30, 2003: PlaceWare 
Feb. 19, 2003: Connectix

The crucial one there was Groove, which Microsoft acquired on March 2005. It seems to have acted as a catalyst for their 'software as a service' strategy - Groove CEO and now Microsoft CTO Ray Ozzie has been a leading light in that. Microsoft's acquisitions history suggests they strip off the technologies and human assets from their acquisitions and put them into Microsoft products. Not dissimilar to what Yahoo and Google do nowadays too.

I'd say Yahoo's key acquisitions over the past few years have been Overture (search), Oddpost (email), Ludicorp/Flickr (social software street cred). You can definitely see a pattern to all of Yahoo's purchases, because they usually get eventually re-branded and folded into the Yahoo business as part of their wide and deep product line. Overture products for instance are now Yahoo! Search products. Oddpost has morphed into Yahoo's Gmail competitor, the new-look AJAX-driven Yahoo! Mail. And Flickr's presence is being felt across the board, in products such as My Web 2.0 and Yahoo 360.

Google is a bit harder to figure out, because as Adam Rifkin noted a couple of months ago, Google tends to buy "small, creative, engineering-driven teams with no-bullshit cultures and interesting products and/or innovative technologies". Maybe the difference comes down to semiotics, as Ben Barren wrote:

"Yahoo's M+A semiotics read : fun, jagged, dangerous in the same way a rollercoast ride is; delicious, flickr, konfabulator (sadly renamed) Google's scrip semiotics are more complex, obtuse, asexual + asynchronous : Android, Akwan, Urchin, Keyhole."

Whatever the outcome of all these acquisitions, expect more to come in 2006. Already people are talking about a Microsoft/Newsgator deal, Yahoo/Netflix, Yahoo/CNET, Yahoo/TiVo. And what about Google/Feedburner? (a shock pick suggested to me by one of my correspondents).