The people at Familysearch.org have created a program where they have uploaded actual images of primary documents. One of these projects is images of the marriage licenses, returns or other record for the entire state of Arkansas. Not all documents are yet scanned but many are. I'm not quite sure why they start their index at 1837, as there are marriage documents extant from before this time, the earliest ones evidently are from 1808 which is prior to when Arkansas became a state. I'm willing to do a search in the earliest records (from 1808 to 1837) for a small fee, just contact me at email@example.com
Arkansas County Marriages, 1837-1957
In 1889-91 the Goodspeed company of Chicago published several volumes in a set of Biograhical and Historical Memoirs of Arkansas. The entire set covered the state, but was broken into regions due to the size of the set. I believe there are six volumes in the set. The only place that I know online has all six volumes, is at ancestry.com. Portions of several volumes have been extracted here and there on certain county genealogy websites.
Biographical and Historical Memoirs of Southern Arkansas, Goodspeed 1890 ($$ at ancestry.com)
Includes the counties of Ashley, Bradley, Calhoun, Chicot, Clark, Cleveland, Columbia, Dallas, Desha, Drew, Hempstead, Howard, Lafayette, Lincoln, Little River, Miller, Nevada, Ouachita, Pike, Sevier, and Union.
Biographical and Historical Memoirs of Central Arkansas, Goodspeed 1889 ($$ at ancestry.com)
Includes the counties of Pulaski, Jefferson, Lonoke, Faulkner, Grant, Saline, Perry, Garland, and Hot Spring Counties
Biographical and Historical Memoirs of Northwestern Arkansas, Goodspeed 1889 ($$ at ancestry.com)
Includes the counties of Benton, Carroll, Crawford, Franklin, Madison, Sebastian, and Washington Counties
Biographical and Historical Memoirs of Northeast Arkansas, Goodspeed 1889 ($$ at ancestry.com)
Biographical and Historical Memoirs of Western Arkansas, Goodspeed 1891 ($$ at ancestry.com)
Biographical and Historical Memoirs of Eastern Arkansas, Goodspeed 1890 ($$ at ancestry.com)
Other online and offline genealogical resources for Arkansas
Jan Davenport states : "I have started posting a collection of Arkansas obituaries to a blog. Most are from Arkansas Baptist newspapers. They are posted in their entirety unless noted. You can access the blog at: http://jan-davenport.blogspot.com/ "