Why not to use a pop-up window

If you're using a pop-up window on your website, you're doing it wrong.
It's akin to using nested tables for display, fer chrissakes!

Reasons why it's wrong:

• It's a poor user experience because the user doesn't get any of the usual browser functionality in the pop-up window, like the back button or the icon that tells you the page is loading.
• I'm sure I'll think of some more later

Outlook 2007 reminder windows always on top

Hit Alt-F11, navigate to Project1 | Microsoft Office Outlook Objects | ThisOutlookSession

Simple way 

Private Sub Application_Reminder(ByVal Item As Object)
    If TypeOf Item Is AppointmentItem Then
    MsgBox "Message text", vbSystemModal, "Message title"
    End If
End Sub

(source)

Sophisticated way

Private Declare PtrSafe Function FindWindowA Lib "user32" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Long

Private Declare PtrSafe Function SetWindowPos Lib "user32" ( _
ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
ByVal X As Long, ByVal Y As Long, ByVal cx As Long, _
ByVal cy As Long, ByVal wFlags As Long) As Long

Private Const SWP_NOSIZE = &H1
Private Const SWP_NOMOVE = &H2
Private Const FLAGS As Long = SWP_NOMOVE Or SWP_NOSIZE
Private Const HWND_TOPMOST = -1

Private Sub Application_Reminder(ByVal Item As Object)
Dim ReminderWindowHWnd As Variant
On Error Resume Next
ReminderWindowHWnd = FindWindowA(vbNullString, "1 Reminder")
SetWindowPos ReminderWindowHWnd, HWND_TOPMOST, 0, 0, 0, 0, FLAGS

End Sub

(source)

Best Android calendar notifications

Options:

• Google calendar (built in) - notification icon only stays visible for duration of "event", so not suitable for reminding you to do something, if you don't check your phone at the right time, you won't see anything. Notifications are only visible if you pull down the notification drawer.
• isoTimer - notification stays there, but app has a slightly weird UI and is slow.
• Touch Calendar - ?

Pop-up notifications on linux

Alternatives:

• notify-send hello # works, but disappears too fast, and can easily be missed
• growl # advanced, haven't figured out how to use it
• knotify # apt-get install kde-baseapps-bin
• zenity # zenity --warning --text "meeting is now"

Hook one of these into your calendar application.

Or for IRC, using pidgin, try one of:

• Pidgin GNTP
• Growl plugin

Or write your own pidgin plugin like I did, to use a notification system of your choice:

• github - pidgin plugin

Add-ons for Thunderbird

• Lightning = a calendar
• Mailbox Alert = pop up a message for important emails (first set a rule to copy them to a folder, then right click on the folder to set the Mailbox Alert)

Pop up a message box in Ubuntu

Try gmessage, xmessage or zenity (in that order)
But none of them force the window to always appear over other windows.