ShareNotice plugin -- basic buttons per-notice to share the text & a link to the notice on other sites.

Default settings list Twitter, Facebook, and Identi.ca as targets.

Using icons built-in, and no magic offsite JS or anything so it won't slow down or break if third-party site goes down.
Default styles are a little limited, but can be customized in theme should one be so inclined.

plugins/ShareNotice/ShareNoticePlugin.php

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+<?php
+/*
+ * StatusNet - the distributed open-source microblogging tool
+ * Copyright (C) 2010, StatusNet, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @package ShareNoticePlugin
+ * @maintainer Brion Vibber <brion@status.net>
+ */
+
+if (!defined('STATUSNET')) { exit(1); }
+
+class ShareNoticePlugin extends Plugin
+{
+    public $targets = array(
+        array('Twitter'),
+        array('Facebook'),
+        array('StatusNet', array('baseurl' => 'http://identi.ca'))
+    );
+
+    function onEndShowStatusNetStyles($action) {
+        $action->cssLink('plugins/ShareNotice/css/sharenotice.css');
+        return true;
+    }
+
+    function onStartShowNoticeItem($item)
+    {
+        $notice = $item->notice;
+        $out = $item->out;
+
+        $out->elementStart('ul', array('class' => 'notice-share'));
+        foreach ($this->targets as $data) {
+            $type = $data[0];
+            $args = (count($data) > 1) ? $data[1] : array();
+            $target = $this->getShareTarget($type, $notice, $args);
+            $this->showShareTarget($out, $target);
+        }
+        $out->elementEnd('ul');
+    }
+
+    private function getShareTarget($type, $notice, $args)
+    {
+        $class = ucfirst($type) . 'ShareTarget';
+
+        return new $class($notice, $args);
+    }
+
+    private function showShareTarget(HTMLOutputter $out, NoticeShareTarget $target)
+    {
+        $class = $target->getClass();
+        $text = $target->getText();
+        $url = $target->targetUrl();
+
+        $out->elementStart('li', array('class' => 'notice-share-' . $class));
+        $out->elementStart('a', array(
+            'href' => $url,
+            'title' => $text,
+            'target' => '_blank'
+        ));
+        $out->element('span', array(), $text);
+        $out->elementEnd('a');
+        $out->elementEnd('li');
+    }
+}
+
+abstract class NoticeShareTarget
+{
+    protected $notice;
+
+    public function __construct($notice)
+    {
+        $this->notice = $notice;
+    }
+
+    public abstract function getClass();
+
+    public abstract function getText();
+
+    public abstract function targetUrl();
+}
+
+abstract class GenericNoticeShareTarget extends NoticeShareTarget
+{
+    protected function maxLength()
+    {
+        return 140; // typical
+    }
+
+    protected function statusText()
+    {
+        $pattern = _m('"%s"');
+        $url = $this->notice->bestUrl();
+        $suffix = ' ' . $url;
+        $room = $this->maxLength() - mb_strlen($suffix) - (mb_strlen($pattern) - mb_strlen('%s'));
+
+        $content = $this->notice->content;
+        if (mb_strlen($content) > $room) {
+            $content = mb_substr($content, 0, $room - 1) . '…';
+        }
+
+        return sprintf($pattern, $content) . $suffix;
+    }
+}
+
+class TwitterShareTarget extends GenericNoticeShareTarget
+{
+    public function getClass()
+    {
+        return 'twitter';
+    }
+
+    public function getText()
+    {
+        return _m('Share on Twitter');
+    }
+
+    public function targetUrl()
+    {
+        $args = array(
+            'status' => $this->statusText()
+        );
+        return 'http://twitter.com/home?' .
+                http_build_query($args, null, '&');
+    }
+}
+
+class StatusNetShareTarget extends GenericNoticeShareTarget
+{
+    protected $baseurl;
+
+    public function __construct($notice, $args)
+    {
+        parent::__construct($notice);
+        $this->baseurl = $args['baseurl'];
+    }
+
+    public function getClass()
+    {
+        return 'statusnet';
+    }
+
+    public function getText()
+    {
+        $host = parse_url($this->baseurl, PHP_URL_HOST);
+        return sprintf(_m('Share on %s'), $host);
+    }
+
+    public function targetUrl()
+    {
+        $args = array(
+            'status_textarea' => $this->statusText()
+        );
+        return $this->baseurl . '/notice/new?' .
+                http_build_query($args, null, '&');
+    }
+
+}
+
+class FacebookShareTarget extends NoticeShareTarget
+{
+    public function getClass()
+    {
+        return 'facebook';
+    }
+
+    public function getText()
+    {
+        return _m('Share on Facebook');
+    }
+
+    public function targetUrl()
+    {
+        $args = array(
+            'u' => $this->notice->bestUrl(),
+            't' => sprintf(_m('"%s"'), $this->notice->content),
+        );
+        return 'http://www.facebook.com/sharer.php?' .
+            http_build_query($args, null, '&');
+    }
+}

plugins/ShareNotice/css/README

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+icon-sharing.png is from http://www.openshareicons.com/
+
+Shareaholic has made the Open Share Icon freely available for use by others under the
+Creative Commons Attribution-Share Alike 3.0 Unported License.
+
+
+icon-twitter.png is from http://twitter.com/favicon.ico and distributed under fair use
+icon-facebook.png is from http://facebook.com/favicon.ico and distributed under fair use
+icon-statusnet.png is from http://status.net/favicon.ico and distributed under fair use

plugins/ShareNotice/css/sharenotice.css

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+.notice-share {
+    width: 24px;
+    float: right;
+}
+
+.notice-share li a {
+    display: block;
+    width: 16px;
+    height: 16px;
+    background: url(icon-share.png) no-repeat;
+}
+.notice-share li.notice-share-twitter a {
+    background-image: url(icon-twitter.png);
+}
+.notice-share li.notice-share-facebook a {
+    background-image: url(icon-facebook.png);
+}
+.notice-share li.notice-share-statusnet a {
+    background-image: url(icon-statusnet.png);
+}
+.notice-share li a span {
+    display: none;
+}