What does stack and what doesnt

so, with the Ulduar instance, getting all the benefits in raid is once again quite important so... im curious what does stack and what doesnt ?
for example,
Imp. judgements (3% crit) with druids FF (same 3%) and / or shadow priests 3% crit
could someone competent please list these things, or direct me somewhere where they are listed ?=)

thanks in advance
-Hut