Marriage Records

Index to Marriages
This has a large list of entries for the years 1849-1863 and 1890-1895. These were transcribed by Greta Thompson.
A-G male surnames, H-P male surnames, and R-Z male surnames
Additional early marriage records contributed by Sharyl Ferrall. These are organized by groom's last name, not by year (though it is given).
Iowa, Marriages, 1809-1992
Part of the collection at FamilySearch.org - searchable by name and free access (no actual images available yet).
Even more records
Many more marriage records completed by Ernie Bradia. Some of these marriages occurred between 1900-1915.
Various Marriages
Another collection of various marriages from 1878-1894 contributed by Constance Diamond. These data have more details.