Logan Rockmore’s excellent menubar notification utility, Mail Unread Menu has been updated and now offers much more control for users over the mailboxes it will track.
Mail Unread Menu works with Leopard (which will be handy) and also features a new exclusions list in its preferences where you can specific mailboxes for the app to ignore.
This is a concentration boost of the first order. For example, in the screenshot below, I have added the mailboxes for two mailing lists:
Because Mail Unread now ignores, it only reports emails that might actually need a response from me, not emails that prefer to browse when it suits me.
Mail Unread Menu is donation-ware and available from Logan’s web site 
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MailTicker has had the ability to check for mail on multiple accounts for years. MailTicker also lets you delete emails that you don’t want to read and/or know are spam. MailTicker is also free.
Check it out here: http://www.sebastian-krauss.de/software/
Sebastian no longer updates MailTicker, and there are reports of its unreliability when running on OS X 10.2 or greater, but it works just fine for my friends and me.