Check out Pool and SnapDomains.
GoDaddy's backorder service is a complete rip-off.
Getting a domain once it's dropping is very hard, as I'll outline below
To give an idea of how things work:
When a domain drops, a company like SnapNames or Pool will try (and usually succeed) in snapping it up (as long as there's demand for the domain).
If only one person wanted the domain, that person gets the domain. However, if multiple people wanted the domain, SnapNames and Pool then auction the domain off and the highest bidder wins it.
However IF there's no real demand for the name (i.e. no-one "backorders" it via SnapNames or Pool), it'll then drop.
Assuming that domain tasters* don't get the domain, a service like GoDaddy's backorder service will try and get the domain.
HOWEVER: there's another disadvantage. If the domain *was* registered at GoDaddy (and it subsequently drops), GoDaddy will auction the domain off at their auction site called TDNAM (The Domain Name AfterMarket) at a very cheap price - either $5 or $10 ($10 at first, then if no-one wants it after 7 days, the price falls to $5 and is auctioned off again).
If, after it doesn't sell at TDNAM, GoDaddy's backorder service will then kick in and attempt to get the domain for you (of course, all registrars offer backordering services; hence it's no guarantee that you'll get the domain even then).
So overall, as you can see, getting a domain after it's dropped is pretty hard. However backordering services are a complete rip-off - if the domain is a good one, go to SnapNames or Pool and give that a shot
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* = a "domain taster" is usually a large company that tries to snap up domains. They then test out how much traffic they get. If they get decent traffic, the company keeps the domain. If it's not a great domain, they return the domain.