See
http://www.lumicon.com/store/c/18-Filters.aspx and click on filter guide at bottom of page.
See also
http://www.televue.com/engine/TV3b_p...4#.U-m_zOOSy5I
See also
http://www.astronomik.com/en/visual-filters.html
See also
http://www.thousandoaksoptical.com/nebula.html
See also
http://www.dgmoptics.com/DGMfilter_portal.htm
See also
http://www.bintel.com.au/Accessories...8/catmenu.aspx page 2 and 3
A UHC is the most versatile and useful for emission nebulae. An O-III is excellent for planetary nebulae and for extra contrast on emission nebulae.
A H-Beta is for the elusive horsehead nebula and a few other objects. Do not buy this one first!
They are also called nebula filters.
I would recommend a UHC type filter. I prefer Lumicon but there are many good ones.