EQ is the most popular. Version 7 was recently released.
http://www.electricquilt.com/
However, you can sometimes pick up an earlier version for much less (EQ5) to see if you like it enough. For figuring out blocks and fabrics and measurements, including applique, quilting designs and thread colors, EQ5 is more than sufficient.
For even less money, there is a very "lite" version of EQ here:
http://electricquilt.com/Shop/QDW/QDW.asp
I've never used it so I don't know how "lite" it is. But the price is right.
This software also has a lot of fans.
http://quiltpro.com/
And there are some less expensive (or even free) programs out there. I'm sure someone else will list them.