Let's say you desperately need to chat with a friend in another time zone as soon as he's awake or at work. Leaving voice mail or email is fine, but what if your friend doesn't think to check voicemail or read email first thing? Assuming he uses iChat sufficiently to log in automatically, you may be able to rely on a little-known feature of iChat: single-use, per-person alerts.
Select your friend's name in your Buddy List, choose Buddies > Get Info (Command-I), and in the Info window, choose Actions from the Show pop-up menu at the top. Then choose Buddy Becomes Available from the Event pop-up menu, and choose an alert - I prefer spoken text. Make sure to select "Perform actions only next time event occurs" if you don't want to be alerted every time your friend becomes available. Click OK, and wait. As soon as he appears online, iChat alerts you, and you have a chance to touch base right away.
I looked for a similar feature in Skype, but as far as I could tell, it has only the capability to alert you in general when any one of your contacts becomes available, rather than when a particular person becomes available (iChat can do that too). Still, unless you have a vast number of contacts, even that general capability could be worth using on a short term basis.