Since I joined ONE Agency at the beginning of this year, I switched to a Mac. I had always been a home-mac-user, but for over 15 years I have spent my days on a Windows machine. So one could say that I have Windows and especially Excel and Powerpoint in my fingers. I'm one of those guys everybody would call upon in case of MS Office questions.
Over the past months I've been experimenting a lot with MS Office alternatives. This is of course due to my Mac-switch, but mainly because we do not (yet) have a stringent policy on which desktop systems. We simply try to work together as efficiently as possible. So we need some 'common ground' to be able we can work on each-others documents.

1. MS Office: The incumbent leader
The office suite is obviously candidate N°1. Great product, everybody knows how to work with all elements, and the specialists know all the work-arounds to solve small shortcomings (e.g. including an Excel sheet in Powerpoint in a practial way = paste as image). If you're working on Windows, Office is the obvious choice. But let's be honest, Word is over-sophisticated, Outlook is not fun to work with and to closed a system to be the center of your life and Powerpoing is boring (Excel is great, no complaints there).
The biggest problem however is that the whole packages is very expensive. Especially for a growing company. Who want's to pay 25 full licences for this ... even if e.g. a designer only use Word every now and then to open a file from a customer.
Collaboration: perfect if everybody uses Office

2. Open Office / Neo Office (for Mac): The open source challenger
Neo Office, the open source version of Office for Mac, offers Excel, Word, Powerpoint and some more stuff all in one application. The functions are 95% identical and the user interface is almost as good as the latest Office suite. So if your life does not depend on super-duper spreadsheets with pivot tables and over 25 connected sheets, and if you're not an editor working in Word every day, Neo Office is perfect.
The fact that it's an open source product also means you need to install updates an patches manually, that may be a problem for some users. But hey, it's free!
Side remark ... Open Office is in fact a functional copy of MS Office, so I feel a bit like we're stealing from years of Microsoft work here ...
Collaboration: Opens and saves Office documents perfectly

3. Keynote and Pages: The Apple alternatives

Apple has their own products for presentations and word processing pre-installed on all Macs. I tried a bit of "Pages" and I must admit it offers some nice features. But nothing that could seduce me into switching though.
Keynote is a different story. It's so much more fun and good-looking then Powerpoint that I'm now turning into a Keynote aficionado. Although I must admit there is some work to be done on some usability things. All the pop-ups clutter my (small) MacBook screen, and to change the appearance of a piece of text you need to open two different windows. But the 3D-effects, lovely shows (very much appreciated my colleague Jo, now nicknamed Joe Shadow) and the possibility to export to PDF, PPT (for sharing with Powerpoint users), HTML, Flash(!) and Quicktime(!!) largely compensate that.
Collaboration: Opening and saving to Office documents is possible, but you loose the nice bits in Keynote.

4. Google Docs: the product of the future
Last but not least is Google Docs and Gmail. I have centered all of my personal organization around Gmail. All my accounts (business, private, hobby, etc.) point to one inbox, and there I organize my life with a "Getting Things Done" plugin. This experience made me a real fan of "everything on the web". Give me somethings that connects and has a browser (PC, PDA, phone, TV set, PSP, ...) and I have all I need for a day's work. Especially because I also use Google Calendar which integrates perfectly with all iCal compatible tools (that's everything except Outlook).
Based on that experience, I started using Google Docs more intensively. The word processor does a good job for simple texts and is great for sharing, but I really need to be able to use nice-looking templates to publish docs for my customers. Google spreadsheet is also OK for simple calculations and lists, but after some time I always run into the limits and need to switch to Neo Office. I did not use the presentation tool Google just launched, but I'd be surprised if it would beat the Keynote experience.
In general Google Docs also still has too many small 'glitches'. There are too many small hickups while working to really get into your document and fully concentrate on the content and not on the tool you are using.
BUT, I'm still convinced this is the future. A small example: My 10 year old daughter prepared a document for school on Google Docs which allowed her to work on it at home, at school and while she was on holiday at my aunt's. All without here having a PC of her own.
Collaboration: perfect if everybody switches (which will not work for now)

So, what is going to be?

E-mail: Gmail
Agenda: Google Calendar
Word processing: Neo Office
Spreadsheets: Neo Office
Presentations: Keynote