All that is required is your web browser and an internet connection. Granted, not all of these webtop offerings are the same, and each has different offerings:
2) Goowy – A web OS that offers 2Gb of email storage, messenger, calendar, address book, News/RSS manager, file sharing & 1Gb storage, games, and widgets.
3) Youos – Intended to act as a virtual computer with common options like a control panel, desktop, system settings amd more.
4) Eyeos – Open source webos that you download and implement on your own server.
5) G.ho.st – the Global Hosted Operating System that uses virtual computer ideals
6) Desktoptwo – A flash based webos offering email (not sure how much storage), address book, file storage, 1Gb sharing, IM, blog, music player and a neat website editor.
7) Glide – From media files to documents - including photos, audio, and video. Also has a word processor and calendar.
8) Xin – More like an complete operating system than a basic webos. Expect big things from Xin.
9) Orcadesktop – A webos in beta, offering preconfigured schemes from XP,
10) Xcerion.com - software that will let people run the equivalent of their desktop applications in a Web browser.
For online portals, and productivity tools you should try Google Desktop, Zoho, or Thinkfree, which is a free online alternative to Microsoft Office. If you just want to spice up your own homepage, Netvibes, Yourminis and Pageflakes all offer the ability to create customized start pages. These can be customized by dropping in widgets or modules. There are thousands of these readily available for free, or you can create your own.