I'm unaware of free options that work wonderfully. Or paid, but I haven't tried as many of those.

Gmail is online only, though yes, it's very good. I need offline, and I have 7 active accounts; Gmail online isn't a complete solution by any means.

Apple Mail has poor / nonexisting IMAP subscription, and few power-user tools, though it's my favorite desktop client so far. Fast with thousands of emails and 7 accounts, accurate searches across tons of data, integration with OSX's Address Book, and the worst plugin API in existence: existing, but entirely undocumented.

Thunderbird is a load of crap. I really, truly want to like it, but it always leaves a bitter taste, despite being one of the better clients overall. It continually chews on nothing until I kill it, preventing any updates to the account it's spinning on. It slows to a crawl on a mere few-thousand emails. Is far slower to retrieve and display data than Mail (easily 5x slower in the best case). Hinges privacy / security decisions on the wrong data (I had spam recently with a display "from" of "Apple.com" - Thunderbird decided it was actually from Apple, and displayed all the images, despite coming from another source and being in my spam box). Doesn't integrate with OSX's address book, and its existing address book is simply a joke, an insult to their users.

Mutt / Sup I intend to look into more fully, but have a high learning curve (the Vims of email clients), and I want / need HTML email and don't recall if they handle such things well.

What other free ones would you suggest I try? I need OSX and Windows software, I'd love efficient tagging support, and intend to move to certificates for signatures / encryption (and every system I've encountered is beyond an inflexible-PITA for certificates, almost all the way to totally ineffective).