When I talk about portals, I'm talking about an HR portal - not the enterprise portal. Portal is not synonymous with employee and manager self service - portals indicate another level of technology. Here's a good portal explanation from a vendor.
In my mind, there are only a few vendors that are providing this service. Here are my top vendors:
3) PeopleSoft (if you can get Oracle to sell it)
4) ADP (outsourced only)
5) WorkScape (HR only)
Plumtree and SAP offer enterprise portals, but the technology and maturity of the products give it a clear advantage over the other products. PeopleSoft can compete well, but who can buy PeopleSoft these days? For HR only portals, take a look at excellent products from ADP and WorkScape.