I find Press (RSS reader) to be very useful as the official one doesnt do text, which blows my mind as to why not.
Apparently the 5.0 update will include a text RSS reader built in, which would be nice. Until then, ill stick with Press.
PSPTube, IRShell, CaveStory port and gpSP also deserve votes.
And if we're gonna include plugins too, then CXMB, RemoteJoy, and the Screenshot plugin all deserve votes.