Researchers with ancestors from Poznań (formerly the Prussian province of Posen) may benefit from the Poznań Project. The site is a marriage indexing project using parish records from the area between 1800-1899. Volunteers have entered over 800,000 records so far.
The database is searchable by groom, bride, region, time period, and record type (Catholic, Protestant, or Civil). First names can be chosen from a list, which encompasses Latin, Polish, and German spelling for the same name. For example, one can choose "Stanislaus/Stanisław" to return entries for both variants. Regions are listed in Polish, but the site offers a table to view each in German.
My search for Martinus/Marcin Michalak and bride Agnes/Agnieszka in Żnin from 1883-1887 is shown below.
To the right of the pink line is an option to "Get Original Record." Clicking on that button takes you to a screen that shows the archive where the original record is located and the address to write for a copy. Keep in mind that responses will likely be in Polish. Most records can also be found on Family History Library microfilm, and the language used will vary.
Lukasz Bielecki. Poznań Project. Database. http://poznan-project.psnc.pl/project.php : accessed 26 June 2012.