The S425G also has WiFi, uses the $30 plan. WiFi is a great feature for under $100.
In my opinion there are two good choices, 840G and S390G. Both have coverage thru AT&T and T-Mobile, shared. Both have Wifi that can be used without burning your minutes and no other models using the $30 plan have Wifi. Both have Email apps and you can install some Java apps like Opera Mini and Google Maps and games.
840G is a capacitive touchscreen but not a smart phone.
S390G is a blackberry style, with open QWERTY keyboard. Very solid feel but somewhat heavy. It'll fit in your pocket but tends to pocket dial Internet and 911. And the keyboard lock doesn't always work 100%. Each app, except the browser, can be locked with the main password.
My preference is the S390G, so I gave more info about it. 840G is not a phone that I've owned but have tried it out.