Below is a chart that shows some of the most commonly propagated plants.  
SW= Softwood
SH=Semi-hardwood
HW=Hardwood
Softwood cuttings are taken right after a flush of growth.  It's a trial and error thing to make this type of cutting.  It
should have enough time on the growth so it snaps like a green bean, and will not immediately wilt when taken.

Semi-hardwood is taken in summer to early fall.  The lower part of the cutting is hard, but not fully mature while the top
is still soft and pliable.

Hardwood is taken in fall or winter.  The wood from the cutting is fully mature.

It is important that the wood you are taking is the current years growth.  If you look at plant growth it's usually pretty
easy to tell where the current year growth is and the old begins.

These are general guidelines and various authors and web sites have a little different description.
Plant Species
 Time of year to take cutting
 Type of cutting
Japanese Maple...Yes you can
take cuttings from the expensive
small trees
 Mid-Spring or early summer
 SW SH
Barberry
 Fall
 SH
Butterfly Bush (Buddleia)
 Spring though Fall
 SW  SH  HW
Boxwood
 Spring Summer
 SW SH
Camellia
 Late Summer
 SH
Dogwood
 Summer
 SW SH
Smoke Tree
 Summer
 SW SH
Italian Cypress
 Winter
 HW
Blue Point Juniper
 Winter
 SH
Tam Juniper
 Winter
 SH HW
Daphne
 Mid Spring to Summer
 SH
White Potato Vine
 Spring, Summer, Fall
 SW SH
Pyracantha
 Spring to Fall
 SW SH
Lombardi Poplar
 Spring
 SW
Arborvitae
 Winter
 SH
Euonymous, many varieties
 Spring through Fall
 SW SH
Pittosporum, Many varieties
 Spring through fall
 SW, SH
This is what I like to grow.  For soft and semi-hard cutting I use Hormodin #2.  I use  Hormodin #3 for
hardwood.  I've recently started using using Dip 'n Grow and really like it. It liquid rather than powder.
There are other exhaustive lists in books and web sites for types of cuttings and when to make them.
Bridge Street Nursery
Half way between Walterville and Leaburg.
Springfield, Oregon