I helped my younger brother get rid of some persistent stuff from Gator recently. We looked up the application that installed the BHO and uninstalled the full application which also unistalled the BHO.
The application that installed the BHO was a freeware program for DIVX.
Other applications that use BHO's are typically freeware because they attempt to pay for their software one way or another.
THe above mentioned site indicates other associated program sources of these annoyances: "For example Alexa uses a BHO to monitor page navigation and show related page links. GetRight and Go!Zilla use BHO's to monitor and control file downloading. Flyswat, Quiver, Blink, iHarvest, etc use BHOs to extend and control Internet Explorer."
From the same site as above, here are other instructions to remove the problem:
http://www.spywareinfo.com/lopgone.html
Last, as part of a more complicated process, you can rename the dll yourself and if it runs a service or driver you can disable these via the services control panel or the Recovery Console.
-- However, you can't change a file currently in use. So if you are using Windows 2000/XP or NT4, stop the service and disable it, then change the dll file extension.