iPhone/iPod touch: The biggest problem with the iPhone's email client is its lack of search. All My Mail can't compose messages, but its free version lets you search Gmail, AOL or Exchange messages pretty easily.
After creating an account with Attassa (which, for this tester this morning, was a bit buggy), you'll be able to add one Gmail, AOL or (with a plugin) Outlook email account to your All My Mail app for synchronisation and searching; the paid version and a monthly fee gets you more than one account at a time.
All My Mail organises and sorts your mail primarily by contacts and attachments, and is kind of smart about it. It groups together email addresses with similar or the same contact names under one heading, and can recreate Gmail's threaded message view by analysing the messages you're trading back and forth. There's an attachment-only view you can flip to when you need to find one particular Word document from a guy named Bob, and the email search and sorting felt fairly responsive in our own tests.
All My Mail has a somewhat lacking security notice, stating only that all transmissions are SSL-encrypted and that their database is encrypted as well. If you're concerned about third-party access to your email, you might want to consider waiting to see if the 3.0 iPhone update brings in something worthwhile in email search. Otherwise, All My Mail is a neat little utility that can't be your all-in-one mail client, but does make up for one of the iPhone's glaring deficiencies.
Here's the official video tour of All My Mail: