As the tech world gets itself in a tizzy over Google saying its 5 percent stake in AOL might be impaired — basically it may now be worth less than they thought it was — we thought we’d revisit some of the most notable tech writedowns in history. Under U.S. accounting rules, once something’s impaired, it can’t be marked up again, which typically leads to a write-off. The write-offs on our list aren’t necessarily the largest, but we think they exemplify a certain point in time in the fizzy world of technology valuations that’s worth remembering, but not worth repeating.

So far this year we’ve seen one of the largest corporate writedowns ever, that of Sprint-Nextel shaving $29.7 billion in value off its $35 billion merger with Nextel Communications. The writedown led to Sprint-Nextel posting a fourth-quarter loss of $29.9 billion, and the company is still dealing with the aftermath of its push-to-talk love affair. At the time of the merger, cell phone deals were the thing to do, but mismatched networks and customer bases doomed this one from the start.

Another relatively recent write-off occurred when eBay woke up next to Skype two years after it offered up to $4.1 billion for the peer-to-peer voice provider and realized it paid waaaay too much for the privilege. It wrote off $900 million, which is small potatoes compared to some of the other writedowns listed here, but worth mentioning as a case of new technology feeding off an old-school web giant’s desperation.

AOL’s purchase of Time Warner back in 2000 was spectacular both for its $106.2 billion price tag and its almost instant failure once the deal closed. By that point, the tech bubble had burst and in 2002 the combined company wrote down $54 billion thanks to the evaporation of AOL’s value. Even the easy money economy of the previous few years has yet to see a deal of such awesome valuation, something for which we can all be thankful.

The dot-com crash was followed by another wave of writedowns related to the companies that had banked on the Internet infrastructure rush, such as Qwest Communications, which spent $36.5 billion on U.S. West in 1999, only to turn around and write down $24 billion of that in 2002.

I’ll put Google’s possible impairment on here simply because it’s amazing that AOL has managed to convince two companies to overpay for it, even as it does its own dubious buys. Since many financial experts are predicting a new wave of impairment charges, perhaps Google’s taking one of the first steps toward write-offs in the tech sector based on the Web 2.0 boom.

Not only did the FCC decide on Friday to berate Comcast for messing with P2P traffic, but apparently it showed no love to Qwest, either, denying Qwest’s request for forbearance in four cities. Qwest had sought the right to stop charging competitive telecommunications carriers wholesale rates for access to its network in Phoenix, Minneapolis-St. Paul, Denver and Seattle, saying the area was now competitive. Kevin Martin, chairman of the FCC and the carrier’s biggest fan, disagreed. Maybe he’s tired of being the industry’s lapdog.

This tiny bit in this DSL Reports post about baby Bell, Qwest, caught my eye.

A recent announcement that the company s CFO has decided to resign also included a quiet statement that business is slowing partly because of the housing crisis that is sapping demand for new hookups .

In my interview with Qwest CEO Ed Mueller back in March 2008, I had asked him, if housing downturn was impacting his business, after all Qwest’s fast growing regions like Arizona and Colorado have been hit hard by the housing downturn. Mueller had said that despite the cyclical nature of the economy, the company doesn’t plan to change his course.

In the other housing crisis Qwest had to take a financial loss when it relocated Mueller to Denver. When Mueller joined Qwest, the company bought his home for $8.9 million in September 2007, and recently sold it for $7.1 million. That’s a $1.8 million hit, thanks to the housing crisis.