Depends. There's push email support with WiFi enabled phones on everything from
JAVA MIDP (feature phones), to
Symbian, to
Android. It just depends on what features beyond that are desired. Keyboard? Form factor? Touchscreen?
Try out
this tool, keep your focus on GSM850/1900 devices, and stay away from Blackberry as Blackberry requires the additional cost of BIS which pretty much is impossible to find in the MVNO world. If you want BB functionality, Symbian S60 is the place to land, and my personal go-to, but it means keeping inside the GSM provider camp (AT&T, T-Mo and respective MVNOs). As such and on that front, it's really hard to go wrong with any Nokia if that's an option. If you need a CDMA carrier (Verizon, Sprint and their respective MVNOs), Virgin Mobile used to have a couple Blackberry models, but your only realistic and easy to find option will be an Android handset. Typically, one doesn't need push email if they only have set times to sit down and check their messages anyway, but it can be a nice convenience... though it just eats data. My wife quite enjoys her
Nokia E63 for what it's worth, and we picked it up in fine condition off Ebay for under $70.
Beyond that, thumb through the
Superguide for ideas.