Using a standalone email client is not, admittedly, something that I do. I have gotten too accustomed to Gmail, frankly, and I don't really see the need for a client like Mail. I imagine you will all make fun of me when Gmail crashes hard someday, while you work with your email offline with a standalone client.
If you are using an open source email client, then you will undoubtedly be using Mozilla Thunderbird.
Like Mail, Apple's default messaging client, Thunderbird has a lot of features going for it. For instance, like Mail, Thunderbird immediately recognised the mail server for my Google Apps for Your Domain address, and even gave me the initial choice to choose between IMAP or POP3 delivery, which Mail did not.
But there were some concerns. IMAP syncing was done on a folder-by-folder basis, wile Mail just grabbed all my messages at once. Now, admittedly, Mail's methodology took forever to sync with my Gmail account. But it was more than a little frustrating to click a Thunderbird folder for the first time and then discover that its contents hadn't been downloaded yet.
For the security minded, remote content (like images) was turned off by default, something Mail left on. Filtering was easy and worked without a hitch.
Thunderbird is a solid standalone mail client, but it has a generally unpolished feel, as it had no access to any of Lion's features, such as full screen mode.
GIMP has long been the go-to open source program for those who need the functionality of Adobe Photoshop, which itself was once one of Mac's flagship programs. Naturally, I had to see how GIMP looks on OS X today.
There's two ways to describe GIMP on OS X, a schism brought about by the GIMP's dual nature on this platform.
First, and absolutely foremost, GIMP handled like a dream. All the functionality is there, and the application practically flew across the screen. Script-fu scripts I have used in the past worked perfectly, and there were no performance differences at all between GIMP on this or other operating systems.
But, in the aesthetic view of a long time Mac user, this is one darned ugly application.
That's because, in order to work, GIMP needs to have the X11 libraries running in the background. That means that GIMP has the look and feel of an X11 app that's been customised to look as much like Aqua as possible. While a long time Linux user like me had no problems with this, I can see where an OS X veteran might freak out a bit.
They shouldn't. As usual, GIMP is a well tuned, feature-rich app that performs very well.
There are many open source applications available for OS X, these are just the start. For now, there seems to be an effort to catch up with Lion's specific feature set, but even the apps that had a different look and feel with the interface still ran well and were very stable.
Open source, then, is evolving right along with OS X.