When it comes to picking the right dropout for your new rig, there are many choices. I have come up with a small selection of some of the more popular options. I prefer to use Paragon's dropouts as they are of very high quality and made in the USA. However, there are a vast number of other styles out there that can be used.
The DR2010: Easily the most common MTB dropout. If you want a straightforward frame with disc brakes this is the dropout to go with. It comes in plain steel perfect for painting or powdercoating and is also available in stainless steel. The only real drawback to these is that it is pretty difficult (but not impossible) to run a rear rack.
The DR2030: Similar to the DR2010 these low mount dropouts are for geared bikes. One of the main advantages to this style is that you can run a rear rack and fenders with existing eyelets. These also come with a removable derailleur hanger and are in both stainless and regular steel.
The DR2007: Your basic road or cross dropout for when you want to run rim brakes. These come in several different angles, with or without eyelets for racks/fenders, and can be made from regular or stainless steel. You CAN use a disc tab with this however there is no real advantage as you have to use a brace from the two stays.
The DS1004: Probably the best single speed option for any type of off (or on) road riding. With a variety of different inserts for various axle and disc brake options you really can't go wrong with these. In regards to setting up chain tension they are the best "set and forget" single speed option out there. They also have to option to split the seatstay at the dropout to allow for running a belt drive. Using these limits you to a top tube housing route.
The DS1030: The other single speed alternative to the sliding dropouts is paragon's low mount rocker dropouts. Where the sliding dropouts mount the disc brake caliper above the seat stay these mount it between the seat and chain stay. The biggest advantage to these (besides looks) is the ability to run racks similar to the DR2030. Using these limits you to a down tube housing route.
Paul Track Ends: For horizontal dropouts or more accurately "Track Ends" you can't go wrong with Paul Component. Adjusting the chain tension with these ends is super easy with the included knurled knobs.