This is far and away the best free SEF component available for Joomla. I was previously using OpenSEF on http://themebot.com/ and although OpenSEF did a good job, the site had become very slow. I have IRC Maxell's page_cache installed, but for initial uncached page loads and for logged in users, the site was very sluggish with OpenSEF.

sh404SEF is ultra efficient and has many useful features. After installing, there was a massive improvement in page load times and a reduction in mySQL queries. Themebot can now accommodate a lot more traffic and serve pages up much faster than before.

There are still a few small bugs, which the author is working hard to fix. Excellent support! Along with sh404meta, this component is crucial to having a large, highly trafficked site that does well in the SERPS. Give it a try and if you like it, send a donation the author's way so that development can continue.