One of the best OPCs is the Dorset OPC and it also serves as the home page for the online project. It boasts a complete list of Dorset parishes. After the parish name, there is a code (example: [P]). The "P" indicates that a online parish clerk is assigned to that parish; if it is grayed out, the post is vacant and users can apply to be the clerk for that particular parish. The code lists over the list of parishes and provides the code's meaning.
Content is spotty for some parishes, but others offer genealogical data as early as the 1500s to the 1700s. A few parishes require you to contact the online parish clerk to perform lookups for you and still others, have barely gotten off the ground requesting that you return to the site at a later date. Some parishes link to external sites where the genealogical data sits offsite.
There is a "search" function which allows a genealogist to search across all Dorset parishes except for external sites. Three separate search portals comprise the expanded search capability and each is clearly marked on which portal you need to use. This is a bit more awkward than the previous search portal, but greatly speeds up the results process. Dorset OPC also offers an updates and contacts page that keeps you updated on new additions to the site and personnel changes in parish clerks. All in all, it's a very useful site and you can't beat the price.