Before starting, I want to note that I haven't tested the add-ons much on XBMC codename Dharma, since I'm currently using the XBMC Beta codename Eden. With Eden the developers have put a lot of focus on the add-on framework of XBMC, giving add-on developers better flexibility and more options.
The way to install add-ons will be through XBMC's official add-on repository, which is accessed simply by opening Settings within XBMC and selecting Add-ons. There you should be able to Get Add-ons. Some add-ons aren't in the official add-on repository, and as such would require either adding an unofficial add-on repository that offers the add-on, or by downloading the add-on and installing it from zip file.
This should (in my opinion) be the first add-on installed. It's an official add-on from XBMC.org and can be downloaded through the default (official) XBMC.org Add-ons repository. It's found within the, rather obvious, Subtitles category.
After XBMC Subtitles add-on have been installed, you should configure it to fit your needs. The obvious things would be setting your primary, secondary and perhaps tertiary (third) languages. Even though my native tongue is Danish I still prefer having English as the primary language, and Danish as the secondary. That choice is purely personal. I just prefer not having to read badly translated subtitles (the pony named "Bill" in Lord of the Rings were translated to "Jens", just to give a good example).
Then you should configure what Subtitle Services you would like to use for finding the subtitles. As I recall both OpenSubtitles.org and Podnapisi.net are selected by default, and is actually enough for almost all TV episodes and movies I've seen. However as you'll notice, there's several more localized services availabe, f.ex. Swesub.nu and Undertexter.se if you want a better selection of Swedish subtitles.
Under Advanced Options I recommend selecting both the "Manually Enter Search String" option as well as "Display file name in results page". Sometimes you won't be able to see which subtitle is the best match, but by being able to see the original filename (if you haven't renamed it, that is), you can give a good guesstimate on which of the available subtitles are the better match. As for manually entering the search string, is only so you have the option, if the subtitles found isn't even for the movie or episode you're watching, then you can try to manually enter the search string.
The last thing you need to do, before you can actually use the add-on, is to enable it in your XBMC Skin. Go to the XBMC Settings page and select Skin. Now where you go from here depends on your active skin. For the default Confluence skin, you go to Add-on Shortcuts and there you select the Subtitle Add-on to be XBMC Subtitles.
When you're watching a movie or TV episode you can press the OSD Controls button (M-key on a default keyboard) and you can press the Subtitle button, which is a small rectangular icon with a line at the bottom (a monitor with a subtitle at the bottom of the screen).
This is a rewrite the the old Apple Trailers add-on. This is found under the Video Add-ons category. There's not much to tell about this add-on, only that sometimes it's nice to watch a few great trailers :-)
There's not much configuration needed for this add-on either. Bear in mind when setting the resolution, that the higher the resolution, the more you'll probably end up waiting for the add-on to buffer the trailer, before it starts playing. However you can select whether or not it should stream or download, again just bear in mind that downloads can be slow, and that the trailer won't start playing until fully downloaded, if that option is set.
This is a simple add-on for listening to podcasts, not much to write about this add-on (simple and useful add-ons are good like that). It is found under the Music Add-ons category.
It keeps track of played and unplayed podcasts, as opposed to some of the others I've tried, which is why I really like this one. I actually use this, along with TuneIn add-on, instead of having a huge music library.
You configure it by pointing it to an OPML file, which you'll have to create on your own (or you can use my techno/trance/dubstep podcasts OPML).
TuneIn Radio is basically just a directory of net-radio stations and podcasts. What makes this service great, is that it's just a webpage that you can access from any browser, and then save your favorite radio stations. They also have apps for Android, iOS and Windows Phone.
The TuneIn Radio Add-on for XBMC is a recent addition to my list of favorite add-ons, mostly because it's not part of the official XBMC.org Add-on repository. To get this add-on you currently have to download it manually as a ZIP file, and then using the XBMC Add-ons "install from zip file" option.