Better Webmail can support multiple accounts. It looks like an optimizing of the available web clients with a few extra features. Other than a passcode option I don't think it offers much more than Mail. I found it when doing research earlier this week, but wasn't interested enough to download and try it, so you're on your own.
It is just about the only thing I found other than one or two apps for specific services like Google, Hotmail, or enterprise stuff. None of them offer the kind of full features email client that people have been asking for, and most have poor to terrible ratings. The GMail and Better Webmail apps are the only two I remember as having good ratings.