Ancestry.com.au
This is a fantastic site.
Although I've been researching my family's history since 2002, it wasn't until fairly recently (late 2009) that I began seriously using Ancestry. Once I started, however, the benefits rapidly became clear.
Before using it, my assumption (always a dangerous thing) was that it was an online library of sorts. Massive, yes, but ultimately just a collection of documents like that found in any genealogical society across the globe. Useful, once.
How wrong I was.
There are two aspects to Ancestry that make this an ever-changing, invaluable resource for any genealogist. The first of these is the 'how about this?' element.
Ancestry is constantly scanning all of the records it knows about - itself a growing number - and regularly asks you 'is this likely to be relevant to the person in your tree?'. As your tree grows, and as Ancestry acquires new records, there are new questions all the time.
This alone is superb.
The second aspect relates to other researchers who are tracing some of the people who are in your tree. There are a number of ways to connect with these like-minded genealogists, and share (and benefit from) some or all of the information you've uncovered.
This collaboration is entirely optional - you can keep your tree private (whether that's just parts of it, or the whole thing) - yet it can help you to rapidly fill out your research.
As with the regular questions, the sharing aspect makes Ancestry a brilliant resource.
Want to take a look at Ancestry in action? Certainly - here's my own family tree. And to test it out, just swing by the main page and register for a free trial.
Highly recommended.