TinyMap is a lightweight server-side web map, which allows viewing a map of your world in a browser. Similar to Dynmap, but simpler and more lightweight.
Drop it into your mods folder and start the server (or client if you really want, it works but isn't terribly useful). Once you've started a world, it will say Open your web browser and navigate to http://127.0.0.1:8080/, which is the URL you can use to view it in your browser.
If you aren't using the same computer to view the map, you'd need to replace 127.0.0.1 with the IP of your server, and if necessary port forward port 8080.