Memento 

Memento is a clock and a reminder service for Windows. I personally don't use reminder software because I tend to forget even that I have a program installed to remind me of things. How bad is that?

Memento packs some features that could make me want to use it if I had a Windows PC. But first I want to go over its basic features. Memento features a clock which can be made visible at all times. It also has a calendar on its right. Below those two elements is a list of upcoming events, which you need to add to the application. Those are your reminders. For each reminder, you can specify an event that you want to occur when the deadline (or date of completion) is met. The most usual ones are to show a warning, play a sound, etc.

But the most interesting feature is the ability to change and modify desktop wallpapers. For each reminder, you can have Memento change the wallpaper completely to any image you have on your hard drive. This makes for unlimited possibilities on how you want your reminder service to work. The best thing of all is that Memento has the ability to add "stickers" to your wallpapers. So, whenever you have to visit your dentist, Memento can place the text you have added for that reminder straight onto the wallpaper, which is just genius. On top of those features, Mement has several skins that you can download and a strong plug-in engine support.