It's pretty situational...at best.
You'll need to communicate with the DM to know when an effect is fear based as it's a keyword, not a damage type and may not be effectively communicated. But keep in mind, any save ends effect from a fear power is a "fear effect".
There are a lot of creatures that use fear and inflict save ends effects (a quick compendium search for "fear save ends" turns up 600 creatures), not all of these have a save ends effect tied to their fear power. But a number of them do. Again, you'll need to communicate with your DM and have him let you know when a power would qualify as a "fear effect." There is a good chance you've faced something that carries one. If your DM doesn't want to outright say it, talk to her about tipping her hand a bit when the monster is attacking etc. And then communicating that you get the bonus when you're actually affected.
It would be worth combing the monster manual a bit and identifying creature types that often carry fear effects. I can identify dragons right off hand, but there will be other types.
That said, this is a minor racial trait, the halfling's main racial punch comes from Second Chance.