Have a look at the Lumicon filter guide here:
http://www.lumicon.com/astronomy-acc...d=1&cn=Filters
It will tell you all you need to know about them, nebula, colour, etc., their features and applications.
From there you can look at a few brands, like Lumicon, and Baader and GSO. GSO are very cheap, but I have no experience with them. If you do most of your observing from light polluted skies, a genral purpose nebula filter. Other than that, the Oxygen III, Hydrogen Alpha and Beta and Ultra High Contrast filters are more specialised as they are suited to different types of nebulae as some are reflection and others emission, so they glow at different wavelengths, hence the specialization in filteration.
I'm looking at getting my hands on a UHC and OIII, but once I finish my current project. I already have a Lumicon Deep Sky filter.
However, you need to know what you are doing as they really drop the intensity of the image. You may be dissapointed at first, but the details are there. Be patient.
One or two colour filters might also help with the planets. An 80A blue will help bring up the Great Red Spot on Jupiter, as it is a rather faint-salmony colour which can make it difficult to see. Check out the chart, and then only select ONE or TWO, no more. You won't use them too many times.
There is currently a discussion on this topic in the Eyepiece and Filter forum:
http://www.iceinspace.com.au/forum/s...ad.php?t=53994