Task.fm takes commands like "meet with Jim tomorrow" or "Replace fish tank filter in 21 days," and converts those commands into future reminders. The reminders will be emailed or sent to you via SMS. The email reminders are free but the SMS reminders require credits with the service—100 messages cost $8. The fee structure isn't outrageous, but we're in agreement that nothing sounds as good as free.
The language engine does have some shortcomings, as well. It doesn't parse commands like "every other" or "next Monday," which makes it less convenient for creating repeating reminders. Regardless of those language hiccups, Task.fm accepted the majority of our test reminders without a problem. If you have a favored service for generating email or SMS-based reminders, sound off in the comments below.