<apex:repeat value="{!params}" var="param">
<apex:dataTable value="{!map[param]}" var="opp" width="100%" align="left">
<apex:column >
 <apex:outputText value="{0, number, $#,###.00}"> <apex:param value="{!opp.Amount}" /> </apex:outputText>
</apex:column>
</apex:dataTable>
</apex:repeat>

Error: The value attribute on <apex:outputText> is not in a valid format. It must be a positive number, and of type Number, Date, Time, or Choice.

Does anybody know why the amount cannot be displayed formatted? {!map} is a Map<String, List<Opportunity>>

It doesn't work for a date, too.

Maybe it's because a map cannot remember datatypes so every field of the opportunity is a String if you access it in Visualforce?!?