tagedit.js

ko.bindingHandlers.tagValue = {
    init: function (element, valueAccessor, allBindings, viewModel, bindingContext) {

        var replaceTags = function(taggedString, targetUrl, linkClass) {
            if (!taggedString) {
                return null;
            }

            var exp = /(^#|\W#)[\w-_]+/g;
            var match = taggedString.match(exp);
            if (match && match.length > 0) {
                for (var i = 0; i < match.length; i++) {
                    var tag = match[i].trim();
                    if (tag != "#") {
                        var uri = " <span class='atwho-inserted' data-atwho-at-query='" + tag + "'><a href='" + targetUrl + tag.replace("#", "") + "' class='" + linkClass + "'>" + tag + "</a></span>";
                        taggedString = taggedString.replace(match[i], uri);
                    }
                }
            }

            return taggedString;
        };

        var tags = allBindings.get('tags');
        var tagUri = allBindings.get('tagUri');

        // set current value
        var value = valueAccessor();
        var valueUnwrapped = ko.unwrap(value);
        element.innerHTML = replaceTags(valueUnwrapped, tagUri + "?tag=", "tag");

        $(element).atwho({
            at: "#",
            data: tags(),
            insertTpl: '<a class="tag" href="' + tagUri + '?tag=${name}">#${name}</a>'
        });

        $(element).on('paste', function (e) {
            e.preventDefault();
            var text = (e.originalEvent || e).clipboardData.getData('text/plain');
            document.execCommand('insertText', false, text);
        });

        $(element).on('keydown', function (e) {
            if (e.ctrlKey && (String.fromCharCode(e.which).toLowerCase() === 'b' || String.fromCharCode(e.which).toLowerCase() === 'i' || String.fromCharCode(e.which).toLowerCase() === 'u')) {
                e.preventDefault();
            }

            if (e.which == 13) {
                e.preventDefault(); // suppress enter key
            }
        });

        $(element).blur(function () {
            var value = valueAccessor();
            value(element.textContent);
        });

        ko.utils.domNodeDisposal.addDisposeCallback(element, function () {
            $(element).atwho('destroy');
        });
    },

    update: function (element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called once when the binding is first applied to an element,
        // and again whenever any observables/computeds that are accessed change
        // Update the DOM element based on the supplied values here.

        var tags = allBindings.get('tags');
        $(element).atwho('load', '#', tags());

        var tagUri = allBindings.get('tagUri');

        var value = valueAccessor();
        var valueUnwrapped = ko.unwrap(value);
        element.innerHTML = util.replaceTags(valueUnwrapped, tagUri + "?tag=", "tag");
    }    
};

Usage:
<div contenteditable="true" data-bind="tagValue: Description, tags: $root.tags, tagUri:'/your-url-to-tags-page/'"></div>