Suggestions:
1. See what your package manager has:
a) Just go into your distro's package manager (for example, "Yast, Install software" under OpenSuSE, or "Software Center" under Ubuntu)
b) Search for "reminder"
c) You should find lots of applications and/or desktop applets
<= Your built-in options will be distro- and version-specific
http://suseroot.com/installing-removing.php?install=CD
http://www.psychocats.net/ubuntu/installingsoftware
2. See what's already installed:
Go to your distro's "control panel" or "start button" (for example, "KDE Control Center" under OpenSUSE) and look around
<= Again: your options will depend on your distro
http://www.yolinux.com/TUTORIALS/GNOME.html
https://help.ubuntu.com/6.06/kubuntu...ktop-tips.html
3. Google for alternatives:
EXAMPLE: "linux reminder applet"