Most audio enthusiasts are smart enough to let someone else do the design work. It is a lot of work to get to the point where you know what you are doing, and the chances are what you want has already been done and already posted online. So if your focus is getting the best results for the least money and effort, this list is for you.
First, the usual suspects
Zaph Audio >
John Krutke has measured many drivers for distortion and published many competent designs. Few will go to the effort of learning how to design a crossover like he can. You will also find the results of various driver tests and distortion measurements. Essential reading for any diy enthusiast.
Troels Gravesen >
You'll find a vast array of designs with considerable variety, including efficient, open baffle and wide baffle designs. Certainly different to what you will typically find at a hifi store.
Tony Gee >
Well designed and documented, catering to a number of different flavours of hifi. If you are looking for some high end speakers with exotic drivers, there are some projects that might appeal.
Curt's Speaker design works
Some of these designs will appeal for their budget conscious nature, others for their novelty. Definitely worth a look.
It's important to keep in mind that some designs are very well sorted, while others could be the work of a first-time speaker builder. There may be some designs you find in these links that could be better. A little discernment is required.
Parts Express customer gallery >
This is one speaker to watch. The concept is an economical speaker based on a waveguide and efficient pro midbass. A waveguide with compression driver is used instead of a dome tweeter. The efficiency is very high and the sound less coloured than typical horns. Ideal for tube enthusiasts or those who like their speakers with effortless oomph. Also a very good choice for home theatre.