If you live in NYC and are looking to use that old sewing machine of yours that's been lonely in your closet, here are some places to check out for sewing classes:
The Fashion Institute of Technology (FIT) has semester long sewing classes for $213 for in-state people (look for the class label FD 131)! The classes are offered every day of the week and last 16 weeks. This class is on my list of things to do...
Etsy Labs - I just discovered that Etsy's headquarters are in Brooklyn! Etsy is a great site where you can buy and sell handmade items. At their headquarters they have The Etsy Labs, which functions as a community workspace - they offer classes, open crafting times (every Monday!), and occasional events.
Make offers a huge variety of classes, including sewing, knitting, jewelry design, and even shoe making! It's in the Lower East Side.
The City Quilter has an intro to sewing class, in addition to lots of quilting classes.
Sew Fast, Sew Easy also has all levels of sewing classes. Some of them seem a little over priced though.