There's a lot of options and in fact I struggled through the same question a while back on some of my applications. And for our different products we ended up with different solutions. So you have to ask yourself
- Are all of the singleton icons on the map of the same "kind" - same shape and color?
- If they're not, do they all live on 1 layer, or multiple layers?
- If on multiple, are you going to cluster each individual layer, or cross-layer clustering?
- If clustering individual layers, what if icons overlap across layers, are you going to have cluster of clusters?
- Do you need to know "what" type of thing is being clustered, or just that "hey, there is A cluster" on the map. And above you said you don't need to know how many of the items are there just by looking at cluster icon.
Here's a couple of examples and what they mean and how they're done. All are done with a custom clustering algorithm, not with the Bing clustering (1st image) or OL Clustering strategy (2nd). This way I have a lot more control over the look and feel.
Screen cap from a Bing app; we have multiple layers of different icon types and colors. We chose to cluster the icons, then hide all by the top-most (most-important) icon in the cluster, and then the top-most icon is overlaid with a transparent-ish image. So say my icons are 20x20, the cluster indicator is a 30x30 image that's 80% transparent except it has a PLUS on the upper right. So when overlaid on top of my cluster "representative" icon, it looks like I have a cluster of things underneath. When user hovers or clicks, the event goes to the cluster icon and they get a message "N number of clustered items" and can click or drill down for more info.
In this situation we took a simpler approach. Icons still live on different layers and have different meaning and we cluster cross-layer, however the representative icon is just a big fat PLUS sign [that varies in size up to a certain limit].
So basically, we chose a "PLUS" "+" to indicate a cluster in both apps, but took different routes on how to put it on the map - overlay existing map icons to give the map more meaning, or just clean up the map and put a PLUS and let the user drill-down for more info.