What are some good webapps that can replace desktop apps?

I'm looking identify what web application are viable alternatives to commonly used desktop software like Microsoft Office, Photoshop, and so on. Some of these are free and make it easy to access your documents, content, or functionality from any machine. While most don't offer all the features of desktop software, they seem to be good enough to substitute for most casual users.

A few great webapps I've found that are starting to displace my desktop tools include:

• Sliderocket (an alternative to Microsoft Powerpoint)
• HomeStyler (an alternative to Punch! and other home layout apps)
• Aviary (an alternative to Photoshop)
• Acrobat.com (an alternative to the desktop version of Acrobat)
• Mint.com (an alternative to Quicken)

What other web apps are people using that are ready to replace desktop apps that do the same thing?

From web app LBushkin

• Zimbra

  (an alternative for Outlook)
  

  Simon Brown : You should post them in separate answers so they can be voted on separately.

  Alex : Done. I moved Google Docs in a different answer.
  From Alex

• Pixlr

  Replacement for Photoshop

  From taylor

• Google Docs

  (an alternative for Microsoft Office)

  From Alex

• Coderun

  An IDE for web development

  From Alex

• Github.com, a replacement for older SVN systems.

  From lostInTransit