I see a lot of people saying that Yahoo!'s stock price is going to suffer because of this, which actually isn't true.
Before the announcement of the intended purchase, YHOO was trading at $19 per share. Since Microsoft was essentially offering $29 per share, a lot of people started buying YHOO at $29. Now that there is no impending purchase, YHOO will go back to $19.
What's changed is the rationale for the perception of the company's value. During the time of the proposed merger, the value was perceived as $29 per share because that was what YHOO shareholders could expect to get from Microsoft (about 0.95 MSFT shares per share of YHOO, plus $15 per share of YHOO). Now that the merger is off, purchasers will once again be basing their value expectation on Yahoo!'s revenue, price-to-earnings ratio, etc.
I will be VERY surprised if YHOO trades much below $19 on Monday. If you can do any after-hours trading this weekend, it would be a good move to sell some YHOO short.