I tend to use the free Reader yBook(
http://www.spacejock.com/yBook.html) on the PC.
While I've tried out uBook, yBook does most of everything I need, so it remain my reader of choice for Html, txt, rtf, etc.
On my Pocket PC I use an old app called Lister (
http://www.crisp.demon.co.uk/pocketpc.html) . I've been using it for years for HTML and Txt files, and being quiet lightweight I can leave it running without it hurting the performance of an old device.
For protected files, I do use MS Reader & Mobipocket, But I tend to prefer to ConvertLIT or DeDrm them and use the unsecured Html files.