That page you show assigns a group to each marker it outputs (it's doing this from an XML file which I haven't looked at).
The following jQuery function is then bound to the checkboxes' onclick event;
function toggleGroup(type) {
for (var i = 0; i < markerGroups[type].length; i++) {
var marker = markerGroups[type][i];
if (marker.isHidden()) {
marker.show();
} else {
marker.hide();
}
}
Here's an example of the checkbox markup...
<input type="checkbox" id="restaurantCheckbox" onclick="toggleGroup('restaurant')" />
This is called with a group name and loops through the markers in that group showing any markers that are hidden.
So the main trick is assigning a group to your markers as you output them and that's basically it!
EDIT.
Here's the XML for the marker data. You could do it this way by
<markers>
<marker name="Pan Africa Market" address="1521 1st Ave, Seattle, WA" lat="47.608940" lng="-122.340141" type="restaurant"/>
<marker name="Buddha Thai & Bar" address="2222 2nd Ave, Seattle, WA" lat="47.613590" lng="-122.344391" type="bar"/>
<marker name="The Melting Pot" address="14 Mercer St, Seattle, WA" lat="47.624561" lng="-122.356445" type="restaurant"/>
<marker name="Ipanema Grill" address="1225 1st Ave, Seattle, WA" lat="47.606365" lng="-122.337654" type="restaurant"/>
<marker name="Sake House" address="2230 1st Ave, Seattle, WA" lat="47.612823" lng="-122.345673" type="bar"/>
<marker name="Crab Pot" address="1301 Alaskan Way, Seattle, WA" lat="47.605961" lng="-122.340363" type="restaurant"/>
<marker name="Mama's Mexican Kitchen" address="2234 2nd Ave, Seattle, WA" lat="47.613976" lng="-122.345467" type="bar"/>
<marker name="Wingdome" address="1416 E Olive Way, Seattle, WA" lat="47.617214" lng="-122.326584" type="bar"/>
<marker name="Piroshky Piroshky" address="1908 Pike pl, Seattle, WA" lat="47.610126" lng="-122.342834" type="restaurant"/>
</markers>