The spreadsheet included with the iWork software suite is called Numbers.
Apple's Numbers software is the spreadsheet component of their iWork suite.
If you ask them and pay for it local Apple stores will sell you a MacBook with iWork preinstalled but only a demo version of iWork is usually supplied with a new Mac.
A 30 day trial version of iWork can be downloaded from Apple's website (See links below).
One can download iWork from the Apple App Store. The latest version was released in 2009. However, Apple did release iWork for the iPad in 2010 as three separate applications.
A 30 day trial version of iWork '09 can be downloaded from the Apple website. (See links below)
I don't think there is a version of iWork for Windows. However, with iWork '09 now going online, it is possible that Apple will be letting Windows users use the service.
Yes, you can purchase Keynote separately without having to buy the entire iWork suite. Keynote is available for individual purchase on the Mac App Store or the iOS App Store. However, it is often included for free with new Apple devices, so users may find it pre-installed on their devices.
iWork 09 is installed into it's own iWork 09 folder in the Applications folder. It should not touch an iWork 08 installation.
There are apps for clipart that are available for free. One of these apps is Clipart for iWork and iWeb available at the Apple store.
IWork was created on 2005-01-11.
All previous iWork files are readable by late copies of iWork but upon opening them you have to save them to the new format of that version of iWork. For example an iWork '06 document opened in iWork '08 will have to be saved as an iWork '08 document upon opening it, which means you will not be able to open it in a previous version of iWork