There are also Windows-based tablets, which will run pretty much any Windows software including the full Microsoft Office applications. Windows 7 has some fairly robust tablet features built in, although it generally works better with a stylus than with a touchscreen. The most common option for Windows Tablet PCs is a "convertible" laptop that has a touch/stylus compatible screen that can rotate around to cover the keyboard. There are also slate (no keyboard) Tablet PCs available from several manufacturers, including HP, Asus, MSI, and a few others. They do tend to be somewhat more expensive than the iPad or various Android tablets, but they could be a better fit if you really need to use full Windows applications.
While Windows isn't as touch-friendly as iOS or Android, it is definitely very usable and it is certainly vastly superior if you want to do handwritten notetaking or stylus-based artwork. Even for simple tasks like web browsing or other similar kinds of content consumption, having the touchscreen is really nice. I've had a convertible laptop for a couple years and really can't imagine considering a new laptop that doesn't have a touchscreen.
Windows 8, which is scheduled to come out next year, has substantial design changes that are intended to make it much friendlier for touchscreen tablets. Once it comes out, it is very likely that there will be quite a few slate tablets available from major manufacturers that run the OS. Windows 8 tablets are expected to compete much more directly with the iPad and Android tablets and, thus, will be more competitively priced than Windows tablets today. There still is some open question as to whether the lower-cost tablets will be fully compatible with older Windows software or not, as they are expected to be built on ARM processors (like in the iPad) instead of on the Intel-type processors found in current Windows systems.
As others have noted, there are apps for most major tablets (and mobile phones) that will let you view and edit Word documents, but you should be aware that their compatibility is somewhat limited. They usually cover the most common features of Word, but if you work with documents that have really complex formatting or which use very advanced features such as macros, those apps won't likely be able to display or edit them accurately. One of the biggest risks is that most of those apps will strip out Word features that they don't understand when re-saving the documents. To be clear, these problems don't come up for most users, but they are something to at least be aware of, particularly if you are a power-user of Word.
You also mentioned the Amazon Kindle Fire. It is important to realize that it is very directly intended as a content-consumption device rather than for content creation. QuickOffice will probably work on it (the app is already available in Amazon's App Store), but there are some potential concerns. Probably the biggest is that the Fire won't have Bluetooth or any other way to easily hook up an external keyboard, which is pretty essential for any lengthy word processing tasks. It also has a smaller screen (7" versus 10") than the iPad, which can limit its suitability for document editing as well.