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How do I change the default email client in Safari on the Mac to Gmail?
Mailto tags currently open in Eudora, but it's time to move to Gmail.

Update (2016): find up-to-date advice in this question on Ask Different: How can I use Gmail as the default mail client app in Mac OS X?

The remainder of this answer (written in 2009) is obsolete, as Google Notifier for Mac is no longer supported.

Here's one way:

1. Install Google Notifier for Mac
   


2. Open Mail.app, go to Preferences -> General, and set "Default email reader" to Google Notifier.app. (Yes, you need to set this in Mail even if you don't want to use Mail...)

Works great for me. And in general I can vouch for Google Notifier too. It's a nice little app that adds an icon (like this: ) in the menu bar and notifies you (if configured to do so) about new mail in your Gmail box.

GmailTo

Gmailto No. 2 is a little app that
allows you to set Gmail as your
default email client in Mail.app, so
that any email link you click opens a
new email in Gmail’s web-based
interface.

Download it, put it wherever you like
and then open Apple Mail’s General
Preferences. Select Gmailto (2) from
the list like this and you’re all
done:

As you can see from the screenshot,
two Gmail notification utilities —
GmailStatus and GmailNotifier — allow
you to do the same thing, but if you
don’t want to use either of them, this
little app will get the job done.

Edit: Apparently this app was created in 2004. I found a Greasemonkey alternative.

But I also found a mirror!

There is also a Safari extension (and Chrome extension) called mailto: which also offers the same functionality.

• Chrome: http://code.google.com/p/mailto-chromeextension/
  


• Safari: https://extensions.apple.com under Email (or direct install)

Unlike some of the other solutions posted here this still works as of 2013.

I used Webmailer years ago, when I was on Tiger. I’m not sure if it works with Leopard, but back then it worked pretty well. You could use it to set pretty much any webmail as the default mailto client, not just Gmail.

If you simply want to use GMail within your default browser in the regular fashion, the other answers you've already gotten are the way to go. If you'd like to run GMail in its own application wrapper (with some nice extra OS integration features), though, I'm glad to suggest Mailplane. It's awesome. You can then set it as the default mail client like any other email app.

In 2019, you can use Open In Webmail to make Gmail open whenever you click a mailto: link in Safari.

In years past, Google Notifier did this (now cancelled), the mailto extension worked (stopped working with Safari 12), there was GmailTo (site and mirror are dead), and Webmailer (no release since 2012).

To use Open In Webmail —

1. Install from the App Store.



2. 
   

   Set Open In Webmail as your default mail app in Apple Mail's Preferences.

   
   
   

   Open Mail. From the Mail menu, select Preferences. Choose Open In Webmail as the default email reader.

   
   
   

   If Mail asks you to sign-in to an account and won't let you get to Preferences, follow this guide to get past it.

   
   
   

   

   
   


3. 
   

   Launch Open In Webmail and select your webmail service.

   
   
   

   

   
   



4. Test using sample mailto links from here.

The above works for Yahoo, Outlook, Fastmail, and other services, too.

For Chrome or Firefox, see this answer for a way to do this without installing any extra apps (though the above works for those browsers, too).

Disclosure: Open In Webmail is my app. I wrote it when the mailto extension stopped working.