familysearch.org (provided by the LDS), freebmd.org.uk, freecen.org.uk, and the Lancashire OnLine Parish Clerk project. The data in the last three are all provided by an army of incredibly generous volunteers who have spent hours transcribing records so that others can have access to them.
It was perhaps fortunate for me that most of family lived in Lancashire - the Parish Clerk projects in many other counties are not nearly so comprehensive as the one for Lancs. I also found the New South Wales Government State Records online service and the Genealogy SA online database very helpful in researching my Australian ancestors. Sometimes I discovered useful information simply by Googling a person's names and dates.
Over time I became quite adept at squeezing as much information as I could out of the commercial sites' free indexes. But often I found that I just couldn't confirm the leads I had without purchasing some pay-as-you-go credits to access the full database. Eventually I decided that life would be a whole lot simpler if I forked out the money for a subscription. Most sites cost around $20-30 per month for a basic subscription, which is the cost of a couple of magazines or a weekly coffee. It's certainly much easier to do the research if you can click on links to your heart's content without thinking "there go another 5 credits" if some turn out to produce irrelevant information.
Ancestry.co.uk because it seemed to be the most comprehensive, and didn't ask for separate subscriptions to access the Australian site. But I still sometimes find information on other sites and use pay-as-you-go to purchase it. (David Whybrew's army records, for instance, could only be found on FindMyPast.co.uk.)
Most of the commercial sites also offer other benefits, such as being able to upload your family tree so that you have access to it online (and make it available to others if you choose to), the chance to connect to other people doing research on the same ancestors, and 'hints' about possible links to your tree (though sometimes these seem to be quite ridiculous and clearly churned out by a computer programme without being checked to see if they are reasonable.) Many offer these options even with a free account. Some also provide the ability to create all sorts of charts and booklets (MyHeritage.com is one example of a site that does this.)
Overall, I could probably have reduced the amount of time I spent researching quite a bit if I'd taken out a subscription earlier. Having said that, the amount of genealogical information available online is growing all the time, and I could also have shortened my research time if I'd started five years later. I'm full of admiration for those who researched their family history on foot and by mail before the internet made it so much easier.