A quick post just to have written one, it's been awhile...
HabitRPG is a to-do application unlike many others. It gamifies your task list by adding Role Playing Game elements. You create a character, you have HP, XP, (eventually) MP and gold. There are three categories of tasks: habits, dailies and to-dos. They're colored from red (bad) to yellow (neutral) and blue (good). Not completing dailies will cost you HP and turn them red, leave them incomplete for too long and your character will die and lose a level. You gain XP and coins by completing tasks. You can use your MP for certain special abilities, for example strike hard at a task and shift its color more towards blue. You can use coins to buy rewards, either self-made or thought-up by the HabitRPG developers.
You can also start a party and go questing with friends, or join a guild. There are also challenges, which are sets of tasks specified by someone else, as a challenge.
I've tried many a to-do application. I've even tried writing my own a few times. I've never really been satisfied. For a long while now I've been using org-mode for Emacs, both because it is Emacs and because it flexible enough to change completely to your own needs. The only problems remaining are identifying your needs and keeping up with your task list. Both are tricky to me, but that last one gets worse the bigger my task list gets.
Unexpectedly, HabitRPG's rewards and random loot are stimulation
enough for me to keep completing tasks. I've been using it for a
couple of weeks now and I'm still completing stuff, which is quite
unusual for me. And I put lots of stuff in there, such as "Drink
water", which is a habit I want to stimulate; or "Exercise" (three
days a week as a daily), which is something I've been needing to do in
a long time and so far I'm keeping it up well; or single tasks like
~/projects", which I've yet to do.
I suppose it helps that I've always liked computer RPGs, and this wouldn't work if I didn't feel that the reward of being able to buy a new weapon for my character is any kind of motivation.
Anyway, if you have trouble motivating yourself to actually complete tasks on your to-do list and it sounds like fun to you, you might try it.