Just wanted to mention, that PRS+ is available for Sony PRS 505 (expect it soon to be ported to 600/300), it allows to:
1) Folder support
2) Book History
3) Bind keys
4) Disable card scanning and carry tens of thousands of books
5) Localization (Catalan, Deutsch, Czech, English, French, ქართული, Русский, Spanish, 简体中文), adding support for a new language comes down to translating
this file
6) Dictionary, Customize UI a bit, games (Sudoku, 5 balls, MahJong)
7) custom epub styles + switching between them
Check
user manual for more info.