Joomla! Users Group Greater Seattle (JUGrGrS)

for a great tool for validating your site's HTML and XHTML, and http://jigsaw.w3.org/css-validator/ for validating your site's CSS.  Both tools have saved me many times when I had an unclosed DIV or other tag that I couldn't find.
 
3) Slideshows are done using RokSlideshow from RocketWerx (http://www.rocketwerx.com/).  Very easy, you just create a text file the same name as each image and it pops up those smooth captions.
 
4) Sitemap (both the HTML version that is linked from the bottom menu and an XML version in robots.txt for Google) is created by the Xmap component (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,3066/Itemid,35/).
 
5) CustomContent module (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,4175/Itemid,35/).  This is useful for sticking an article in any module position.  It is very similar to the built in "CustomHTML" module, with the important difference that all of the normal Joomla backend stuff runs for it (plugins, etc.).  For example, suppose you put your email address in a module position (as I do in the upper right corner of my front page).  Using the built-in CustomHTML module, your email address is there for every spambot to grab, but using CustomContent, the email gets cloaked in Javascript just like emails in articles do by the Email Cloaking plugin built into Joomla.
 
6) Missing Metadata module (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,2846/Itemid,35/).  This is a backend only tool, but it gives you a list of all of the articles for which you have not specified keywords or a meta description.  Google and other search engines display what you put in the meta description when one of your pages comes up in a search, so it is important that these are filled in with nice text.
 
7) GoogleMaps plugin (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,1147/Itemid,35/).  This is providing the map on the contact page.
 
8) Joomla Magic Updater (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,6259/Itemid,35/).  This is another backend component that you don't see, but it provides a nice, single-click button to update Joomla when a new version comes out.  I don't know how they do it, but it just works and really should be built in to Joomla.
 
9) eXtplorer (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,2630/Itemid,35/).  This is a pretty heavy component that I will likely uninstall when I launch, but during development it provides the ability to browse and edit all of the files on your site straight from the backend without using FTP or other ways of connecting to your site.  This is very convenient when you just want to quickly tweak a margin size in CSS or that kind of thing.
 
Some components I started out with but later removed:
 
A) sh404sef (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,2380/Itemid,35/).  Almost everyone highly recommends this for modifying page titles, creating search-engine friendly urls (/blog instead of index.php?option=com_content?...), and a million other things it does.  My problem with it was that you can get Joomla to do the most important of these items on its own if you learn how to configure it, and the component adds 2.5MB and every once in a while hits a bug that is very frustrating.  I am a big fan of small very focused components/modules/plugins and tend to prefer installing ten small plugins to do ten things I need done than one huge system-interrupting component that tries to do a lot of stuff I do not need.
 
B) JomComment (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,676/Itemid,35/).  This adds comments to a blog, and looks very nice.  However, it refused to interfere only with my blog and was putting hits counters and badly colored graphics that didn't match the rest of the site on other pages, even after I configured it to be activated only for the blog page.  A little time with the author to get this fixed and I will probably put this back on.
 
C) QContacts (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,4811/Itemid,35/).  This is a replacement for extending the built in contacts functionality of Joomla, and I was originally using it for my Contact page.  Ultimately I found myself to be better off just writing that page as any other article, which gave me full flexiblity on layout.  In addition, I use GoDaddy for hosting and they do not allow mail to be sent from PHP code (which Joomla is) except: 1) by using SMTP with a limit of 250 messages per day, or 2) by using their custom form that will send mail to one address you provide only.  I commend them for trying to stop spammers and I was able to integrate their form into the custom contact article I made, so it all works and I am not hampered by the 250 message limit.
 
D) Joomulus (http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,6493/Itemid,35/).  This creates a really cool flash-based cloud of words used on your site (or in a particular category of articles).  I was originally using it for a tag cloud on my blog but for some reason it was crashing Internet Explorer 7 and it was a bit too flashy for a lawyer site.
 
Another real gotcha with this whole project was browser compatibility.  If you do any kind of template modification I recommend that you test constantly at each stage in IE6, IE7, Firefox, Safari, and Google Chrome.  All can be installed on one computer with the right tricks, and it is amazing that anyone can produce a consistent looking site with how different they all behave.  If you have an existing site like I do, Google Analytics (http://www.google.com/analytics/) is a great free tool for finding out what browser people have been using to visit your site.  This let me know, for example, that 15% of my visitors are still using IE6, so I had to support it despite the extra work that caused me.
 
Another thing I will comment on is how much of this is free.  Of all of the items above, I only had to pay for a membership to the template site (about $75/year) and JomComment (about $50).  Joomla provides an amazing foundation and a lot of value, and the developer community for extensions is excellent.
 
I hope this information helps someone else.  Please let me know if you have any questions.