Show row number in page block table

You may also be able to use the CSS counter syntax to generate the numbers purely client-side:

#myTable {
    counter-reset: section;
}
#myTable .row {
    counter-increment: section;
}
#myTable .row .counter::before {
    content: counter(section);
}

Where "#myTable" is the ID for the table you're generating, "row" is a class for each row element, and "counter" is a class for each counter:

<table id="myTable">
<tbody>
<apex:repeat value="{!rows}" var="row">
    <tr class="row">
        <td>
            <span class="counter"></span>
        </td>
    </tr>
</apex:repeat>
</tbody>
</table>

---

Edit: Here's a fully functional example using a standard controller:

<apex:page standardController="Account" recordSetVar="records">
    <style>
    .myTable {
        counter-reset: rowCounter;
    }
    .myTable .row {
        counter-increment: rowCounter;
    }
    .myTable .row .counter::before {
        content: counter(rowCounter);
    }
    </style>
    <apex:pageBlock>
        <apex:pageBlockTable value="{!records}" var="record" styleClass="myTable" rowClasses="row">
            <apex:column>
                <span class="counter"></span>
            </apex:column>
            <apex:column value="{!record.Name}" />
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:page>

You can use an apex:variable, as shown in the code below, to show a row number.

<apex:variable var="index" value="{!1}"/>
<apex:repeat value="{!yourList}" var="ob">
    <apex:outputLabel value="{!index}"/>
    <apex:variable var="index" value="{!index+1}"/>
</apex:repeat>