Integrating The Online With The Offline

Imagine.  Imagine automatically opening a JPG with Flickr.  Imagine opening a Word document, and getting ajaxWrite.  Imagine making Gmail
your default client.  Well, you can!  Here's how (note: mostly only
works with Windows XP.  I'm REALLY sorry Mac and Linux users.):

Office Files

• ajaxOffice (the Write, XLS, and Sketch tools), will automatically integrate with DOC, XLS, and SVG files with this simple hack. 
  (Note that the tool Lifehacker provides only works with ajaxXLS,
  ajaxWrite support was broken.  Use the tool that I created in the
  link). If you want to try compiling Autohotkey for Linux/Mac from
  source, download the code [C++].
• Zoho Office/Google Docs & Spreadsheets/gOffice users can create their own Autohotkey script based on the ajaxOffice script.

Media Files

• Flickr provides a tool that integrates into XP's right-click menu.  Download it here.
• BlogMatrix's Sparks also integrates into the right-click menu.
• ImageShack's Toolbar for Internet Explorer and Windows XP also works with right-click.

Email Clients

• G-Mailto and G-Mailto for Mac make all mailto: links open in Gmail.  Or you could use the official instructions (requires Gmail Notifier).  KDE users click here, GNOME users click here.
• Yahoo! Mail: Windows Internet Explorer users get an official ActiveX control here.  Firefox users of all platforms go get a Greasemonkey script here.  GNOME users go here.  Mac OS X users get  Yahoo! Mailer here (works with Gmail and Hotmail too!)
• Hotmail
  users on Windows: Right-click on the taskbar, select Start Menu tab,
  click Customize, check the Email button, and select Hotmail/Windows
  Live Mail.  Mac users use the above Yahoo! Mailer.  Not sure about
  Linux users, but if you use Linux you're probably smart enough to
  create a script or something.  Besides, who needs Hotmail?

Misc.

• Killcode's Upload Express works with YouSendIt and ImageShack.
• Box.net supports WebDav, letting you map it as a drive in Macintosh (instructions) or Windows (instructions).  You can also use NetDrive.
• An unofficial way to Send To YouSendIt via right-click.
• About.com has a guide to online backup/storing/sharing services, some of which map as network drives.