Mark Hurst of Good Experience recently relaunched his fantastic todo list tool, Goodtodo (formerly Gootodo) . He addressed some accumulated user feedback and did a spiffy intro/tour page but it basically operates the same as before--which is just fine because it remains clean, stripped down, and dead simple to use.
Here's his tour video:
The feature that I find particularly useful as a bookseller (and one that I believe is unique to Goodtodo) is the ability to forward emails to a particular date.
So say for instance I tell a customer that I'll hold a book for 2 weeks; instead of staring at that email every day--and trying to remember why I kept it--I can just forward it to Goodtodo dated for the day I want to end the hold (ex, Forward to: firstname.lastname@example.org - Subject Line: "End hold on Baptism in Shame").
Voila! Unsightly email is gone until I NEED to think about it. It's also great for tracking booksales, bill payments, customer appointments and the like. When you have any todo items scheduled for a particular day, Goodtodo will email you a list on the morning.
Goodtodo has built in logic for parsing dates so it recognizes "March25" as this coming March 25th or "Wed"/ "Wednesday" at this coming Wednesday (and many other variations) so you don't need to overthink how you address the forward. It also stores the body of the email in a details field so you can easily keep track of the entire email exchange.
Also importantly Goodtodo also has a very clean, simple interface which loads quickly and plays well with your Iphone or smartphone.
Mark is a big proponent of inbox zero--which I like in theory but can't quite get there--and Goodtodo is the main tool in his arsenal. If you want to join him the good fight you can purchase his manifesto, Bit Literacy.
Goodtodo is available at a free/trial level (limited to 10 todos a day) and a full version for a nominal subscription fee ($3 per month). Give it a try and I'm sure you'll find it worth the subscription
Full disclosure: Mark is a friend whom I regularly trounce at Settlers of Cataan and Pirate's Cove.