First, my general bias is against software that needs to be installed. For almost all tasks there are online apps/sites that do the job as well as anything you can download and install. With collaboration in particular, 24/7 access from any Internet-connected device is mandatory. While there are plenty of excellent sites for online collaboration, there are few which offer the "complete package", i.e. a single login for multiple collaborative tasks, such as editing documents, writing blogs, creating wikis, running discussions. Of the few "one login" suites, Google is the king. It is far from perfect and it is not the best in any single collaborative task, but it has all the pieces you may need under one roof. (If you must have a non-Google solution, I mention a few possibilities at the end of this post)
Real-time collaborative editing: Google Docs
Google Docs suite shines as a real-time collaboration platform, supporting simultaneous editing by up to 8-50 people (depending on the application; although any number of participants higher than a handful would result in a bleeping mess). All the participants can see where the other people are within the document, thereby avoiding accidental clobbering of each others' work. Here's a short clip of 2 people working at the same time on a document; the purple word-select (and the yellow bullseye cursor) belong to one editor; the green bar is the cursor of the other editor. You can also see the editors communicating using a chat window on the right side.
In addition to "documents", Google Docs suite also includes spreadsheets, drawings, forms, and presentations.
Inadequacies: Google Docs suite is weak in the organization department. Google Docs' "collection" is a cross between folders and tags which many people find inadequate for document management. Another complaint with Google Docs is lack of offline support -- you need good Internet connectivity to use Google Docs.
Alternatives: none that are good.
Wiki: Google Sites
Google Sites supports all the typical wiki features in a reasonably intuitive package: visual editor for wiki pages, themes and templates, sharing and permissions, inclusion of photos and video. The availability of many gadgets is useful. It is also well-integrated with Google Docs.
Inadequacies: Google Sites does not support simultaneous editing. When editing a page, it is very easy to mess up formatting (e.g. lists, fonts, headings) and difficult to fix it back without resorting to direct html editing (which is painful even if you know html well).
Alternatives: Wikispaces (free), Weebly (free), Wikia (free). (See also Top 10 Sites for Creating a Wiki).
Blog: Blogger
Blogger is simple, intuitive, free, and takes Google login. It is not as powerful or customizable as Wordpress.
Alternatives: Wordpress.
Forum: Google Groups
Google Groups (and its precursors) offers standard forum features with a no-nonsense interface. Some of the alternatives (Spruz and Webs.com) offer a broader social-networking platform where forum is just one of many social features.
Alternatives: (I have not tried these alternatives and cannot speak personally to their quality) Grou.ps, Grouply, Spruz, Webs.com.
Non-Google options
There are a few other well-established collaboration suites. The suites are Zoho, Microsoft 365, and Zimbra. I feel all three are inferior to Google's offering.
For more collaborative alternatives to Google's offering, check out the articles 10 Reliable Alternatives To Google Docs and 7 Coolest Real-time Collaboration Web Apps You Might Not Know About (But Should).
