I'm using Firebird which has description field for all system objects (tables, columns, views, procedures and parameters, triggers, etc.) It's nice because you can easily share it with others (docs go with database, not separately) and you never lose it.
Most admin. tools for Firebird allow you to edit these descriptions and there are some specialized tools (like IBDesc, for example) that create nice HTML or PDF reports you can print (for some or all tables) easily.
