I am originally from Washington State, so naturally one of my favorite online resources is the Washington State Digital Archives.
This is a collection of state records that have been digitized and is available online for free. Records available include (but are not limited to) vital records, including birth, death and marriage records; census records; auditor records; court cases; land records; power of attorney records; and professional license records to name just a few categories. Years covered are different for each type of record and for each county; overall, the years covered range from frontier years to the present.
Of course, free is one of the most attractive aspects of the website, but I like that this is an ongoing project. Not every county is represented, but folks are hard at work digitizing, so from time to time new counties come online. What this means to me is that this website is a returnable resource. You can return every so often to do your fundamental searches over and over - sometimes turning up new information.
For instance, over the years I've located some of my father Earnest Payne's marriage records - one to Jennie Wilson Slagle who became my stepmother. Surprisingly, I located two marriage records to the same woman - recorded in the same year, but six months apart. There is no indication that the first marriage license was never used, so this anomaly remains a mystery for the time being. You know how it is - you finally get to the bottom of one mystery, but your answers raise several more questions and mysteries!
I had always thought that my father married his first wife (first as far as I know!) in Oklahoma (which is where my father was from), but when the Archives added Clark County to their available digitized records, I idly did a search for my father, never expecting anything new to pop up. Imagine my surprise then when his marriage record with his first wife (a Hazel Pauline or Pauline Hazel Jones - my oldest half-sister's mother) showed up - they had gotten married in Clark County, Washington! A woman I had always thought was from Oklahoma now appears to have been raised in Washington State.
This might not seem like a big deal to some, but I strive for accuracy in detail in tree. Of course, accuracy is a lifelong pursuit when you're doing genealogy - your tree is never completely immaculately trimmed and shaped, a la a bonsai tree. Nevertheless, I always welcome a website resource that provides information that brings your tree into sharper, more accurate focus.
That's why I'm recommending the Washington State Digital Archives - not for everyone I know, but to Washingtonians a valuable resource!