Complicated hardly does it justice. It's a very convoluted process given the current design of Visualforce (particularly, the lack of support for Map objects in value attributes). Best I can come up with is this:

Controller:

public class gridController {
public gridController() {
grid = new list<gridrow>();
loadItems();
}

// stores one row of data-- a product name and a list of cells.

public class gridrow {
public gridrow(string productname) { cells = new list<gridcell>(); name = productname; }
public list<gridcell> cells { get; set; }
public string name { get; set; }
}

  // stores the quantity of a single cell

public class gridcell {
public gridcell(integer qty) { quantity = qty; }
public integer quantity { get; set; }
}

// visualforce needs a list in order to operate

public list<gridrow> grid { get; set; }

// Allows us to render a dynamic number of columns.

private date startDate, endDate;

// Get a list of month names, i.e. "July 2010", "August 2010", etc.

public List<String> getMonths() {
list<string> months = new list<string>();
if(startdate==null || enddate==null)
return null;
for(date currentDate = startdate; currentDate <= endDate; currentdate = currentdate.addmonths(1))
months.add(datetime.newinstance(currentdate.year(),currentdate.month(),1).format('MMMM yyyy'));
return months;
}

// load the items into memory.

public void loadItems() {

// organize by product id and month

map<id,map<date,integer>> dateMap = new map<id,map<date,integer>>();
map<id,string> nameMap = new map<id,string>();

  // loop through all the items, organizing them using maps.

for(object__c o:[select id,date__c,product__c,product__r.name,quantity__c from object__c]) {
if(dateMap.get(o.product__c)==null)
dateMap.put(o.product__c,new map<date,integer>());
if(dateMap.get(o.product__c).get(o.date__c.tostartofmonth())==null)
datemap.get(o.product__c).put(o.date__c.tostartofmonth(),o.quantity__c);
else
datemap.get(o.product__c).put(o.date__c.tostartofmonth(),datemap.get(o.product__c).get(o.date__c.tostartofmonth())+o.quantity__c);
if(namemap.get(o.product__c)==null)
namemap.put(o.product__c,o.product__r.name);
if(startdate==null || o.date__c.tostartofmonth()<startdate)
startdate = o.date__c.tostartofmonth();
if(enddate==null || o.date__c.toendofmonth()>enddate)
enddate = o.date__c.tostartofmonth();
}
grid.clear();

// at this point, if we have any records, we create the list that visualforce needs.

if(startdate==null || enddate==null)
return;
for(id prodid:datemap.keyset()) {
grid.add(new gridrow(namemap.get(prodid)));
for(date currentdate = startdate; currentdate <= enddate; currentdate = currentdate.addmonths(1))
if(datemap.get(prodid).get(currentdate)==null)
grid[grid.size()-1].cells.add(new gridcell(0));
else
grid[grid.size()-1].cells.add(new gridcell(datemap.get(prodid).get(currentdate)));
}

 Page snippet:

<table>
<tr>
<td></td>
<apex:repeat value="{!months}" var="month">
<th>{!month}</th>
</apex:repeat>
</tr>
<apex:repeat value="{!grid}" var="row">
<tr>
<th>{!row.name}</th>
<apex:repeat value="{!row.cells}" var="cell">
<td>{!cell.quantity}</td>
</apex:repeat>
</tr>
</apex:repeat>
</table>

Complex? Yes. I expect it will probably take you at least as long to understand it as it took me to create it-- the logic is actually quite simple, but is made complex by the limits of visualforce. Note that if you wanted to sort the products in a particular order, that would require yet more work.

Best of luck to you on your endeavors. Please feel free to message me if you need further assistance.