Mostly using Ancestry, which has been my main source, though the transcriptions can be a bit hilarious at times. Always look at the original documents, though these can be very hard work to decipher and don't necessarily trust what the enumerator has written. I have also used Curious Fox, though had very little feedback from there.
Recently I started looking at Ireland Reaching Out, which I think may be fairly new, haven't seen it in the past. I understand you have to pay to look at pages, though leaving messages is free, hoping someone will contact you. Having two Irish immigrant ancestors made it worth looking at. I knew the area of one family where their surname was quite common, but the other was simply from "Ireland". And only on one census. Elsewhere she is said to be from Durham. Just another of the joys of census research!
Someone from there kindly pointed me towards parish tithe records where you can see the name of the person paying - presumably the father - but if you don't know that it would be a long slog to trace all of those names to perhaps other family given names. And as ever the writing is rather difficult to make out.
I need to get back to Scotland's people now, as I've discovered that one of my Irish family names was originally Scottish. And another family is almost certainly Scots originally too but I need to get back before the earliest records of them in England.
I did find some families on Mormon records, though how accurate they are is anyone's guess.
I've done some hunting on Rootsweb too but following others' trees can sometimes be misleading, with false information. Always do your own checks. Even on Genes Reunited, where my tree is, I have had what I am convinced is wrong information from another researcher. It bore no relation to what I'd found. Names handed down through families is often a good clue to whether this is your family, though obviously not infallible. Certainly for me it was a big help.
FreeBMD can be useful. GENUKI is interesting. And very occasionally looking for a name on Google can produce something.