I though the Roku remote app for Android was pretty cool as a replacement for a lost Roku remote, but the Official XBMC Remote app for Android goes beyond being just a remote control app and really does a great job of tying together two of my favorite pieces of software. The XBMC Remote app is free and is made by some of the devs behind XBMC, so there's no need for any of the unofficial apps anymore.
In order to get this to work you fist have to setup the latest XBMC on your Xbox or computer to allow control of XBMC via HTTP and configure the username and password. If you're using XBMC on an Xbox you'll want to make sure you're running the latest XBMC4XBOX and not an old version. The options for remote control by HTTP are in the Services tab of the Network configuration section of the XBMC settings. Once that's done configure the XBMC Remote app on your Android phone and try it out.
You can actually use XBMC Remote to make your XBMC instance playback content located on your Android phone by using the Samba Filesharing app in conjunction with the Remote app. I think that's kinda cool, using your phone to control an instance of a media player on another device to have it playback content that's stored on your phone.
XBMC Remote can also do cool stuff like display notifications for incoming SMS text messages and even pause your movie for you when you get a phone call. That's why it lists "read contact information" as a needed permission in the Android Market listing. Since it's an open source app by the official XBMC dev team, it's pretty trustworthy, so I don't worry about that.
In addition to letting you control an XBMC instance on your computer or modded Xbox, you can also use XBMC Remote to share content you run across on your phone with XBMC. For example, if I go to a page with a video to download on the Android browser and I click the link while XBMC Remote is installed on that phone, I can go to "Share" and I'll get an option to share the content with XBMC. Basically this will share the address of the content with XBMC, which will then try to play it. You can then switch back to XBMC Remote to pause/play that content, though since it's streaming from the web you won't be able to skip through it.
All in all, the Official XBMC Remote app is an awesome bit of software for any XBMC fan who happens to have an Android phone. Combine it with the Samba Filesharing app or a UPnP Android app, and your XBMC + Android setup is even cooler.