Microsoft has offered a cut-down, ad-supported free Starter Edition of Office 2010 for quite a while now, but it has only been available pre-installed on netbooks and other low-spec PCs. At the moment, however, you can download the whole thing for free from Microsoft's servers.
I use Starter Edition on my travel netbook, and I can't say the advertising (which appears in a permanent right-hand screen pane) has ever bothered me — virtually all of it just suggests upgrading to one of the full versions. You don't get the full Office experience — no Developer tab and no macros, for instance — and there's only two apps included: Word and Excel. But free is a lot cheaper than full-edition Office, which can cost as much as some netbooks if you buy the Professional edition.
If you're already happy using Libre Office or OpenOffice.org, there's no obvious reason to switch to this, and there's definitely no reason to if you already have a "proper" copy of Office. If, however, you use the full Office apps on your main machine and don't have enough licences to go around, then having effectively the same thing on your other PCs can be very useful.
Note that since the link is to a downloadable .exe file on Microsoft's servers, rather than a fully hosted page, its official status is unclear and Microsoft might well pull the download in the future.