Beacon of Light is the Holy Paladin's cure for lacking AOE healing abilities. It's great in heroics and raids. There are two main methods of use:

When you're tank healing:
Put Beacon on the Tank. When AOE splash damage happens and you are healing the rest of the party/raid, your tank gets heals still.

If for some reason you're raid healing and not assigned to a tank:
Put Beacon on yourself. You will stay up and healed while you're taking care of the other healers and the dps.

How it works - You toss beacon on your UberTank. When you heal MageBoy (who is down by 3000 health) with a spell that heals for 5000 health, the EFFECTIVE heal is duplicated onto UberTank. UberTank gets a free 3000k health increase.

When I get home from work, I'll post my macro for dual-boxing with a Holy Pally and using Beacon. It casts the beacon only every 60 seconds, no matter how often you push it. So I put that macro on 3 buttons I use commonly as I tank with my Prot Pally. This way beacon is ALWAYS up.