If you have more knowledge about particular systems, please get in touch so that this page can be updated and expanded.
You may also find this page and this page helpful: they both have quite a comprehensive list of email programs and how to make them send in plain text.
Please try using Outlook-QuoteFix or OE-QuoteFix - this will solve most formatting problems.
Note that it's not (yet) compatible with Outlook 2007. The nearest equivalent if you have that version is QuoteFix Macro, which provides the same basic features as Outlook-QuoteFix.
You should also:
Entourage actually does proper quoting as standard! Do check the options for replying, though, in case something (eg. a company group policy) has broken them.
All versions of Outlook (including Entourage) have an issue with the use of non-ASCII characters, such as curly quotation marks and long dashes. The issue means these characters will often appear incorrectly (or not at all) on non-Microsoft systems, or even sometimes on other Microsoft systems.
The best fix for Outlook is to find "International Options" (usually under Tools / Options / Mail Format), and set the "Preferred encoding for outgoing messages" to "Unicode (UTF-8)". You also have to turn off the "Auto select encoding" option, otherwise the problem can still occur.
Unfortunately, current versions of Entourage don't seem to have any way to make this setting permanent: you have to choose "UTF-8" each time you send an email that might have any special characters in it. This is especially problematic, because MacOS has its own special character set... combined with Microsoft's mishandling of ISO-8859-1, you will end up with a complete mess unless you use UTF-8 every time.
Technical detail: by default, Outlook will label an outgoing message as being in the character encoding ISO-8859-1 ("Latin 1"), even if it includes characters which are actually non-standard Microsoft additions to that encoding (more correctly labelled "Windows-1252"). Forcing Outlook to label the message as Unicode seems to make it follow the standards a bit better. Note that unless you also disable the auto-select option, Outlook will still try to use the label ISO-8859-1 if all the characters of the message exist in Windows-1252, even if UTF-8 is listed as preferred.
The latest versions of Microsoft's webmail system have taken a giant leap backwards in standards compliance, and get formatting badly wrong by default; in some circumstances they cannot even be configured to get it right. In particular, your message will be mangled badly if you attempt to compose in HTML (or rich text) with any part of the original message quoted. If you have the option to switch to the "plain text" composer, please do so (for all messages you ever send to anybody, not just mailing list ones). Bizarrely, this is not always available as an option for all types of account. If you find you cannot switch to plain text then your only option is never to quote any message when replying - delete all included text before starting to type your reply. Note that this issue affects all replies sent using the Microsoft webmail system, not just those sent to mailing lists.
Please see this page for details of how to turn off HTML in AOL email clients and hence get message formatting right. (Executive summary: it's different in each version, and some don't work at all!)