This SPOJ Problems Classification (1) tool provides information about SPOJ problems and associate them with a set of tags.
Also check Facebook note.

This SPOJ Problems Classification (2) tool provides information about SPOJ problems and associate them with a set of tags.
Also check Facebook note.

SPOJ Backup Tool allows to download your submissions to local disk.
Also check github repo, this thread and Facebook note.

SPOJ User Statistics tool shows interesting users stats.
Also check Facebook note.

SPOJ Utilities provides users' stats and more.
Also check Facebook note.

SPOJ Rank Tracker - Firefox add-on that tracks your position in the main SPOJ Ranking.
Also check Facebook note.

bbone - phpBB add-on that allows to execute submissions directly from posts - installed on SPOJ forum.
Also check this thread and Facebook note.