Counting query results and displaying them in visualforce

It is best to create separate classes for things like controllers and data holders so something like this:

public class AssetSummaryController {

    // This inner class just holds the data needed for the page
    public class AssetSummary {

        public String name {get;set;}
        public integer quantity {get;set;}

        public AssetSummary(string name, integer quantity) {
            this.name = name;
            this.quantity = quantity;
        }
    }

    private account acc;

    public AssetSummaryController(ApexPages.StandardController controller){
        this.acc = (Account)controller.getRecord();
    }

    public list<AssetSummary> getListOut() {
        list<AssetSummary> assetList = new list<AssetSummary>();
        AggregateResult[] ar = [
                SELECT Item__r.name name, count(id) quantity
                FROM Serial_Tracker__c
                WHERE Account__c = :acc.ID
                GROUP BY Item__r.name
                ];
        for (AggregateResult resultList : ar){
            assetList.add(new AssetSummary(
                    String.valueOf(resultList.get('name')),
                    Integer.valueOf(resultList.get('quantity'))
                    ));
        }
        return assetList;
    }
}

which then allows the page to look like this:

<apex:page standardController="Account" extensions="AssetSummaryController">   
<apex:pageblock title="Asset Summary">
    <apex:pageblocktable value="{!ListOut}" var="lo">
        <apex:column value="{!lo.name}" headervalue="Name" >
        </apex:column>
        <apex:column value="{!lo.quantity}" headervalue="Quantity">
        </apex:column>
    </apex:pageblocktable>
</apex:pageblock>          
</apex:page>

The error you were seeing was that you had not provided the alias name "quantity" in the aggregate query. I strongly suggest formatting queries on multiple lines so it is easier to spot such omissions.