If you have access to Forrester, they have a wealth of research on portal trends and vendors in this field. IBM provides Websphere as one of the biggest competitors to Sharepoint (a richer portal environment, tho rigidly tied to Java, as MS is rigidly tied to .NET). Our company currently uses BEA Aqualogic, formerly called Plumtree, which supports both platforms well. All three vendors provide powerful, if expensive portal options.
Then there are off-the-shelf close-to-open-source portal platforms like Joomla, Jetspeed, JBoss and others. Some other big players include Oracle Portal and SAP, although they may be a little restrictive for what you want to do.
One thing that will help in your search is evaluating specifically the capabilities you want, the scalability you need, the price you're able to bear (or cross-charge), and how much custom functionality you'll need (and on which platform). Answers to those will help narrow down your list significantly. There's a lot of really big stuff out there, so knowing what you want helps tremendously.