What else?

So, I'm a MediaPortal user. But what competition is MediaPortal up against?

First and foremost, Microsoft. While Windows XP Media Center Edition 2005 was still a separate package, the media center support has been integrated in Windows Vista. You'll have to get the Premium edition though. With the performance requirements that it imposes on your hardware, Vista is not the most cost effective choice for a Media Center PC. In general, MCE only works with certain MCE certified hardware.

Even though MediaPortal is free, Microsoft obviously blows MediaPortal away from a marketing standpoint. I think it's a wise choice for many people as it doesn't require specific installation or tweaking of settings. In fact, it doesn't have a whole lot of settings, you'll just have to put up with the way Microsoft set it up for you.

Meedio is another option. This program was formally known as myHTPC and development was stopped in 2006, after the software was sold to Yahoo! Yahoo! then turned it into Go TV which is only available to the US market. Meedio still has some advantages over MediaPortal, most notably the way you can manipulate how lists from its database are displayed in the user interface. Meedio used to be available for a modest price.

Worth mentioning also is xLobby. According to the website and the reviews it's very customizable and supports all kind of networking features. It's ahead of MediaPortal with features like multizone audio, pocket pc support and multi client support, but less user friendly. xLobby has never been open source, but the website always featured a "Free Forever" claim. However, xLobby went commercial in March 2007. A new paid version is coming out this year.

In the US the market for commercial Media Center packages is really taking off. Options include SageTV Media Center, Snapstream Beyond TV and CyberLink PowerCinema. Some of these packages are really marketed only for the US and Canada. That means that the standard TV guide is not supporting tv channels for potential users outside North America. Optionally, the package includes TV tuner hardware, a remote, etc.

And there's lots more! MythTV is another open source project, but less developed than MediaPortal and running on Linux. The latest version 0.2 dates back to September 2006 and the site doesn't show a lot of activity since.
GB-PVR is a project developed in the UK. Plugins are supported but the core engine code is not publicly available. Some of the screen shots on the site look an awful lot like MediaPortals BlueTwo skin, Would that apply to the source code as well?
France has several projects. Sesam-TV is freeware. Domotix is also a French project, free. Another French project is DomaxCenter. There's a paid and a free version. The focus is on home automation and there's no live TV support. And while we're at it, here's a nice review comparing 6 projects, also in French. Another example of a home automation package is MainLobby. Tvoon is a German project, but the latest release dates back to 2005. Got All Media is another open source project.

For Linux users and next to MythTV, there's Geexbox and Freevo. And for Mac users, Apple has developed Front Row. However, Front Row is so far without support for live TV.

Update: Of course, there's a much more complete list of HTPC frontend software available on internet. See Eirikso's blog.