Apple Mail - my current favourite, especially in Lion.
Postbox - Paid for, but feature-rich and quite polished AFAICT
Thunderbird Free and open-source. Extremely configurable with a similar extension ecosystem as Firefox.
Microsoft Outlook - A very fully featured email and calendar package. It replaces Microsoft's previous Mac email client, Entourage.
Sparrow - A unique email application. The layout encourages casual, Twitter-like conversation by evoking the timeline view of popular Twitter apps. It's pretty trendy at time of writing.
Mailplane - Basically an optimised, dedicated Gmail browser window. Worth looking into if you're a heavy Gmail webmail user.
MailMate - I hadn't heard of this one before, but it has a good review at Lifehacker.
Excuse me for not answering your question, but IMHO it's better and more robust to follow the development via web, not via app.
You should be able to read the whole repository via http/https if your SVN is hosted via the apache svn module.
Another great way to monitor changes is to use some web-based frontend, which follows repository changes. A great tool (which I use and recommend) is Trac.
Sparrow is now available for the iPhone, and it supports IMAP.
The app description does not mention exchange, and it specifically says that there's no POP support, but being that it's such a popular mail client, I figured it's worth mentioning.
Disclaimer: I've never used it either on the Mac or on iOS
Best Answer
Popular email clients for the Mac include: