I've built myself this simple router jig that I have found very useful for my mission furniture projects.

When combined with a top bearing template bit and router, very precision cuts can be made in minimal time with minimal effort. Depending on the task, you may not even need to measure.

It can be made in a couple hours.

Modify the design to fit your own requirements. I'm getting ready to make a second one to fix some of width limitations of my first design. This one can only take 1.5" wide stock. I'd like to make one that can accomodate up to 3.5" wide stock.

I've found it most useful for:

1. Cutting slots for "slat side" projects, where a 1.5" wide slot is cut into a piece of horizontal wood. 1.5" vertical slats are then fit into the slots. (See my mission bookshelf project for an example piece where this is used.)
2. Cutting tenons for mortise an tenon joints.
3. Cutting mortises for mortise and tenon joints. (OK, I cheat. I cut have the slot in two boards, and then glue them together to make a piece with a nice through mortise hole.)
   

The jig was not terribly difficut to put together. I used 1x2 red oak for the rails. During assembly, I placed a piece of 1x2 between all the rails to get them all alligned nicely. You may want to use a different dimension pieces than I did depending on the look and type of work you plan to do with the jig.

The important part for not needing to measure is to have a stop block positioned at the end of the piece you are routing. I screw my jig and stop block to my cheap pine work table, so nothing moves. Once the stop block is properly in place, you just need to slide the piece into the jig until it hits the stop block, make sure it's flush against the side, then clamp it down.

Setting up the depth of the cut is rather easy. I've got scraps of 1/4", 1/2", and 3/4" red oak. I slide a piece in that is at the depth of the cut I want to make, set the router depth so the bit it touching the wood, and it's good to go. In my limited experience with this, that is the way to most accurately set the depth. Plus, it's quick, easy, and doesn't require any measurement.

 I built this to create identical 1x2 rails with three perfectly matched slots to accept 1.5" x 1/4" slats. I was able to turn out the last 4 (of 8) rails in about 5 minutes a piece.

If you make a jig like this, make sure to take extra effort and time to get everything square and very precise. The quality of the jig will have a critial impact on the overall quality of the mission furniture you are able to create using it.