This will allow you to write web pages that refer to those scripts rather than copies on your own site, reducing your bandwidth, but also leveraging the infrastructure capabilities of Google, such as their content distributed network (which means users would be served those files from a location much closer to them), properly compressed, minified, cacheable files, etc.
In addition, if your visitors have been to other sites using the same technique, they would not need to download the same libraries all over again, and with increasingly rich web sites, these files can get quite large.
So, if you use jQuery, you can use this in your web pages:
Of course, this is not for everyone, and it may be worth bearing in mind some concerns raised by Peter Michaux, who also provides a useful way to handle the scenario where you think access to Google from the user’s location might be restricted (e.g. blocked from a paranoid office!):
Still, it could be a useful technique to further improve the download/load performance for a number of sites.
A number of months back, Yahoo was the first to announce something like this when they said they would host their YUI library for you so it is good to see others getting into this.