The recent Ratchet & Clank Collection is missing one game from the PS2 era: Deadlocked, a darker, multiplayer-oriented arena combat game that focused more on gunplay than platforming. As noted on Wikipedia, it is the only game in the series to keep a Teen rating (all the other games have been re-rated E10+).
Developer Insomniac Games announced an HD remake will be coming later this year, exclusively on the PlayStation Network. Presumably, the game will retain (and perhaps expand upon) the online functionality featured in the original PS2 game.
Andrew Yoon posted a new article, Ratchet: Deadlocked HD coming to PS3 later this year.
The recent Ratchet & Clank Collection is missing one game: Deadlocked. Apparently, it's coming as a standalone title to PSN later this year.