I'm looking for a really simple ogg-compatible player. I don't need video playback or fm radio or anything fancy like that. I'd just like to find something that makes it trivially simple to browse a folder full of music files. Any suggestions? Flash or HD based is ok.

iaudio x5 is a tried and true winner for what you need. And better yet, buy it and install Rock Box. An older unit at this point, but it does exactly what you ask.

How's the interface on the x5? Is it pretty easy to navigate & cue up albums etc?

It is easy if you organize your files in different directories. It is a directory browser rather then an album browser.