When I need these kinds of ad hoc reminders, I always try to think of where I'll most likely be when I need reminding. That will then determine the tool or format I use for the reminder. If it's a general, any time tonight, type of reminder, I can set it on my calendar (iCal), and I know I'll be in front of my computer at some point during the night.
Other times, I need a non-electronic physical reminder. For example, we had a series of casual dress Fridays back in October that I knew I wouldn't normally remember (such a creature of habit!) I knew the moment I would need to be reminded was while getting dressed in the morning, so I wrote "Casual Dress" on a sheet of paper and taped it to the shelf in my closet so it hung down in front of my clothes. Worked perfectly.
I guess it's all about the context.
Doug