SplitMail is a jailbreak tweak for iPad that adds access to your mailbox list in portrait mode. Sometimes you just don't want to use landscape mode and since stock iOS limits Mail in portrait mode, this is a great fix.
By default stock iOS hides the mail list when you're in portrait until you've either tapped the All Inboxes button or rotated to landscape mode. Even then your messages don't appear in their entirety off to the right and the minute you tap somewhere else your mailbox list is gone. I am frequently jumping from folder to folder when organizing and sorting messages so portrait mode on an iPad has always been an annoyance to me.
SplitMail for iPad solves this problem in an instant since there's nothing to configure. Just install the tweak, restart your Springboard (you'll be prompted to do so), and then pop into your Mail app. You'll now notice that your mailbox list will stay static on the left of the screen instead of disappearing each time you go into a message or tap outside of it.
It will give you less screen real estate to work with when viewing messages in portrait mode but it's a compromise many will probably be willing to make for a consistent experience. And if you need more room to read or browse an e-mail you can always rotate your iPad into landscape mode.
I've found this feature to be quite useful when my iPad is docked and I don't want to have to pull it off just to sort through e-mails. It's free so what are you waiting for? Give it a try!