Nickname popup alerts are a typical way to catch your contacts' attention, or annoy the hell out of them. You know the little blue notify windows (called toasts) that appear in the bottom-right corner of your desktop when somebody signs in? You can generate several of these alerts simultaneously, all on top of each other, to broadcast a message in a fancy way. The toasts will disappear after a few seconds, but don't over-use this because it can be pretty irritating for your "victim(s)".  Jared S suggested I added the instructions to the FAQ, so in what follows I will describe the old school, manual technique to generate "nickname pops".

A. TARGET ONE INDIVIDUAL

1) Change your display name to the message you want to grab one of your contacts eyes with (eg. I (L) YOU, JEN).
2) Open a conversation window with that person.
3) In the upper area of the conversation window, to the right of the "Fun & Games" icon, you see three round buttons. Click the middle one with the red line over the MSN Buddy.
4) A dialog window will popup asking for a verification. Tick the "Don't show me this message again" box and click OK.
5) Now keep clicking that same button about 8-10 times to unblock & block this contact. Every time you unblock, your nickname message will pop up on his or her screen. Make sure you finish when the contact is unblocked, so he or she will be able to message & respond to you.
6) Optionally, you can change your nickname to something else again.

B. TARGET ALL YOUR (UNBLOCKED) CONTACTS

1) In order to broadcast a certain message to all your contacts, you first have to change your display name to the message you want to grab their attention with (eg. PARTY AT MY PLACE TONIGHT AT 8 PM).
2) Close all open conversation windows.
3) Click your nickname and set your status to "Appear Offline" and then to "Online" again repeatedly (repeat the cycle about 4 times). Stop when your status is Online.
4) Optionally, you can change your nickname to something else again.

C. TARGET A GROUP OF CONTACTS

1) If you don't want to "spam" (or should I say "spim") your complete contact list with a message, but just a few, you can use Messenger Plus! for this.
2) Create a temporary new group (Contacts > Manage Groups) and copy the buddies of your choice into it. For this technique to be succesful, you will have to limit the number of contacts to about 4.
3) Change your nickname to the message you want to spread.
4) Right-click the new group and select Messenger Plus! Extras > Block Contacts in Group and then Messenger Plus! Extras > Unblock Contacts in Group repeatedly about 4 times. End with the group unblocked. As you can see, it takes a bit longer to block/unblock a multiple number of contacts, so this method doesn't always work out well. It all depends on the number of contacts in the group, the speed of your Internet connection and the speed of the .NET Messenger Service.