Look at Windows SharePoint Services before you even consider doing anything else.
The beauty to Windows SharePoint Services is the built in functionality and portal environment that is intended purely for team collaboration and information sharing. It may be slightly more expensive in terms of external hosting but as an optional extra component to an existing onsite Windows server it is truly superb.
If you team it up with FrontPage 2003 you have the most unbelievable partnership right out of the box.
There are loads of portal applications available and I'm not just referring to the likes of phpBB and Invision PowerBoard that are so often used for small community forums.
Choosing one above another is down to identifying your organisations requirements to a very fine point. Failing to correctly identify these system requirements through proper requirements analysis is arguably the one area where you stand to really trip yourself up.
As a 'roll-your-own' example, I was instrumental is setting up a large intranet for the college I lecture at.
It is built almost entirely using FrontPage 2003 and Dreamweaver MX 2004 (they can share webs in collaborative projects) and the whole shebang leverages a mix of ASP/MS SQL and ASP.NET/MS SQL. I used some Visual Studio .NET 2003 Enterprise Architect but the majority of work was done in the two editors mentioned previously.
Hammering out the initial system design, as you can imagine, was the most vital aspect but once that had been done three of us had the skeleton system up and running within a very short period and it has now evolved into a vast system accessed by tens of thousands (internally and externally) and supporting upwards of 2000 IT students, their tutors, and several outside satellite centres connected via a WAN.
Of course, we had staff on hand and, crucially, already on the payroll, who could cope with software engineering on this scale. My point though, is that even a very large Intranet can be generated if you have appropriate staff to hand for relatively low cost. I can't disclose actual details for obvious reasons, but our application saved the college a very great deal of money and does EXACTLY what is required of it, rather than being an off the peg solution that we tried to adapt to our requirements, or vice versa.
To be honest, I doubt any one of use could make any serious suggestions without some pertinent information.
1. How many employees will the application serve ?
2. How many sites (physical buildings) does the business have that need to art of the intranet ?
3. Do you want internally facing, externally facing or a mix of both ?
4. Do you already have dedicated web servers ?
5. If so are they your own, outsourced to a host or are you running an individual hosting account on a per website basis ?
6. Assuming you have the relevant hosting tied down, are you running or intending to run Windows, Linux, Novell or whatever ?
There are loads of key questions I could add to the list but identifying requirements and matching them against an off the peg intranet application is a big, big topic.
I'll help as much as I can with specifics, but there is no way I can possibly suggest this or that product without knowing at least something about the intended environment and available resources you either have or are willing to obtain.
Best regards (and good luck)