I know, I know this has been covered before... But its out of date. Most plug-ins don't work on Mail 4.2 in Snow Leopard.
I am using three at the moment that are all working great and they are all free
:
1. WideMail is a way to view mail in columns (Ala entourage) it also allows for more customization of the message line:
2. Herald works like growl on steroids you can view your mail as it comes in and scroll through the whole message it also allows you reply without having to revert to mail and I think it looks quite good too.
3 Attachment Scanner How many times have you sent an email promising attachment only to forget to attach the document you want to send? Well this little genius plug-in avoids all that by scanning your email message and if it picks up that you wanted to attach something it pops up a warning before you send off the message. It works very well and has saved me a few times...
If you can recommend any other plug-ins (preferably free) then please post them here...
I am using three at the moment that are all working great and they are all free
1. WideMail is a way to view mail in columns (Ala entourage) it also allows for more customization of the message line:

2. Herald works like growl on steroids you can view your mail as it comes in and scroll through the whole message it also allows you reply without having to revert to mail and I think it looks quite good too.

3 Attachment Scanner How many times have you sent an email promising attachment only to forget to attach the document you want to send? Well this little genius plug-in avoids all that by scanning your email message and if it picks up that you wanted to attach something it pops up a warning before you send off the message. It works very well and has saved me a few times...

If you can recommend any other plug-ins (preferably free) then please post them here...