Here are some essential applications to complete your HTPC.
YAMMM (Yet Another Media Meta Manager): If you have any movies downloaded or backed up Windows Media Center is capable of displaying the box art and some nice meta data. YAMMM will automatically download all of this data for you. The only thing that you will have to do is set it up and organize your movies correctly. The setup is that each movie has it's own folder inside of your movie directory. An Example would be C:\Movies\Up\Up.avi. Here are my settings for YAMMM:
Tips: If the movie data isn't being downloaded correctly find your movie first on themoviedb.org and then rename the folder to the same name which is displayed on themoviedb.org.
MyChannelLogos: By default there are no channel logos in the tv guide. However there is a nice application called MyChannelLogos which will automatically set all of the logos for you.
Media Center Studio: This allows you to completely customize the look and feel of Windows Media Center. From removing icons to creating custom themes this application is awesome. Here is a short video of what you can do with it by my good friend Adam:
Flash 10.1 RC4: If you wish to run flash decently on the Revo than you can install the Flash 10.1 RC4 which enables Hardware accelerated graphics. Now you can watch Youtube and Hulu.
Update: If you have read my Hulu Desktop review then you have seen that I have much better success with Beta 1 of Flash 10.1 than beta 2. I have uploaded the Flash 10.1 beta 1 with the active x plugin as well. You can download it here.
Update 2: If you want to give Flash 10.1 RC 1 a try you can download it here.
Silverlight: Used for Netflix streaming inside of Media Center, Silverlight is essential.
Google Chrome: Since the Revo isn't super speedy I highly recommend Google Chrome for browsing. It is super fast, start up instantly and allows for a lot of real estate on the screen.
Video Lan VLC: VLC is by far one of the best media players out there. If there is some type of file that wont show up or play inside of Media Center simply boot it up inside of VLC and I am almost 100% it will play just fine.
Other add ons:
Media Browser: This is a great add on that I used for a bit of time that helps organize all of your content in one place. It shows nice backgrounds and also displays metadata for you videos you have downloaded. Since I have a lot of content I found it easier just to stick with media centers default UI.
Meta<browser>: This is a great application that allows you to find meta data for all of you tv shows and movies that you have downloaded. It will also automatically rename your files to whichever format you like. This is probably my favorite feature of all. I do not use it for movies as Yammm handles that for me automatically. Hopefully Yammm 2.0 will come out and handle tv shows as well.
My Movies: This is another great add on for people with a large amount of movies. Instead of showing only 2 rows of movie artwork it will fill you entire screen and it also displays meta data a lot nicer.