Here is a list of things that most professional home decorators use:
Measuring tape - A retractable steel tape, 25 foot long.
Nail kit: Keep a set of various sizes for all kinds of tasks.
Notebook: A small notebook for sketching and note taking.
Magic hem: Iron-on magic hem from craft stores, that creates seams without sewing.
Glue gun: Keep it handy for many decorating purposes.
Hammer: A must in every household!
Pins: Both straight pins and safety pins (for drapes/curtains).
Screwdriver set: Keep several sizes in your tool kit. Also keep many sizes of screws.
Velcro: For various fabric decorative jobs.
Wire: For hanging picture frames and other smaller decorative items.