RPG Mechanics

Action Table (table determines action outcomes)
Attribute/Stat Based (STR, CON, PER, etc)
Cards (cards drive the action)
Class Based (Pilot, Wizard, Scientist, etc)
Competitive (players compete against each other to win)
Description Based (Narrative more so than Dice)
Dice (Percentile)
Dice (Primarily d10)
Dice (Primarily d12)
Dice (Primarily d2/coin)
Dice (Primarily d20)
Dice (Primarily d4)
Dice (Primarily d6)
Dice (Primarily d8)
Dice (Primarily Fudge/dF)
Dice (Specialized)
Dice (Various)
Dice Pool (Roll multiple dice vs target)
Dice Step (variable dice vs. static target)
'Diceless' (does not use dice or other randomizer)
Exploding Dice (Die may 'explode' into a second roll)
Free-form (minimal or no rules - think improv theatre)
GM-less (No gamemaster needed to play)
Grid-based (uses square terrain for move/combat)
Hex-based (uses hex terrain for move/combat)
LARP (Live-Action-Role-Playing)
Level Based (Earn XP and level up)
Lifepath Character Generation (Character starts at birth)
Physical Object
Play-by-Mail (PBM)
Point Based (allocate points to get skills, powers, etc)
Progression Tree (Skills, professions, magic abilities, etc.)
Race Based (Player Race/Species affects gameplay)
Random Attribute Generation (during Character Creation)
Reading (for Periodicals, Adventure Books, etc)
Roll and keep (4k2 means roll 4 and keep 2)
Skill Based (buy or gain skills)
Trait / Aspect Based (personality affects gameplay)
Unusual Randomizer (not using dice or cards)
Wagering / Betting (wagering during play)