Another system that is less dependant on playing cards but was always good was the "bennies" systems from Savage Worlds. Three ranks of "chips" that gave you various bonuses (in D&D I would hazard +1, +2, +5 for the values).
These were awarded on top of experience to encourage roleplaying and participation above and beyond just playing the piece of paper in front of you.
Advantages: it was more freeform so the DM and players have a little more artistic control over the outcome - say they spent a +1 chip and used it for an attack roll, the descriptive result was in the control of players and DM. In addition the DM could sway the uses of the chip if he/she thought it was going to ruin the game itself.
Disadvantages: It required more artistic control by the players and DM
I've played both systems and they both have merits. What I did find with the Whimsy System was that some players would just sit there reading them and spend a lot of the session "waiting" for the right moment to use them. They ended up not having much fun.
A system I developed which was a combination of both systems was my Tarot System (used in 3.0/3.5 plot driven games). At the start of every session each character would get one Tarot Card +1 or 2 more based on a "luck" roll which was a roll under the average of their stats. If they rolled under they got +1 and if they rolled under half they got another. I found more than 3 was too much.
A Major Arcana was MAJOR - it could change something in a big way (read below). Minor Arcana gave bonuses or slight modifications (like a -2 to an enemies roll etc). You needed a higher number to affect others (even party members).
During the game they could spend these cards (they went away at end of session) to change the plot line somehow. For example in a combat with a powerful enemy in one of his warehouses (side not: the female player's character - my wife actually - had been going on "dates" with a Paladin from the bodyguard's guild in the city) was knocked down and about to die. Just before I got initiative for the BBEG she played "The Lovers". Suddenly through a high window the Paladin bursts through and tackles the guy to the ground giving the party a chance to escape.
She wanted him to turn up and kill the BBEG - I thought that it would be more fun to save her and the party (they still got the XP) but keep BBEG around for a while.
Of course this led to later complications of courting and wooing but it was a blast.
Advantages: very freeform, will not derail a DM's game if he/she prepares well enough.
Disadvantages: requires a lot of quick thinking.