What were some new weapons that were produced and used during World War I?

There were many new weapons that were produced and used
during World War I.  Perhaps the three most important of these were the airplane, the
tank, and poison gas.  Of these, only the tank ended up being very important to the
result of WWI itself.

WWI is best remembered for the
horrors of trench warfare.  This warfare was made worse by the use of poison gas.  This
technology made such an impact on the psyches of the people who saw it that it was not
used in combat in World War II.  The other two inventions became much more important
later on.  Airplanes did not make a difference in WWI, but they certainly did in WWII. 
The tank did help end WWI, but it was refined and used in huge numbers in
WWII.

These three new weapons are very much connected with
WWI because they were first produced and used during that war.