Having trouble with a particular concept? These online
tutorial sites offer interactive instruction and practice.
More of these sites come online every year.  If you find
good ones that I don't list here, please let me know.  GBA

An introductory exercise on 1D motion.

Motion Diagrams

A nice tutorial on Motion and Graphing.

Graphing Motion

The interactive practice exercises within these three topics
are all excellent, but stay away from the DEMOs, as those 
tend to be buggy, and may crash your browser.  GBA

Vectors<> 1D Kinematics<> 2D Kinematics

The "Sum of Forces" exercise in Dynamics, Part D. 
is particularly good. GBA


Here is an excellent Free Body Diagram MC quiz (note the
directions in the FBD's are not always perfect, but that
doesn't really harm the quiz).  GBA

Free-Body Diagrams

Some simple exercises at recognizing forces. GBA

Forces from the Physics Classroom

Here are some explanations at the AP High School Level for 
Momentum and its Conservation. GBA

Momentum from the Physics Classroom

Good, short explanations of the concepts involved in Rotational
Motion.  GBA

Rotational Motion from HyperPhysics

This SHM tutorial is very good.  I found only one error, at the
end of the "Initial Phase" section (the right answer for that
question is either 3pi/2 or -pi/2).  I have one other minor
complaint.  It says "the acceleration is a maximum" when the
acceleration has its maximum positive value.  OK, but normally
the phrase "the acceleration is a maximum" would refer only to
the magnitude of the acceleration.  But in particular the
training in "Amplitude, Period, and Frequency" and "Velocity
and Acceleration" is excellent. GBA 

Simple Harmonic Motion

Here are some explanations at the AP High School Level for Wave
Motion and Sound. GBA

Waves and Sound from the Physics Classroom

Here are some DC circuit basics and some DC circuit MC questions.
(Note that some of the "simulated experiment" links within this
DC Circuit tutorial were broken the last time I looked.) GBA

DC Circuit Tutorial and Self Test

In the DC Circuit Tutorial there are some broken links for
simulated experiments using Kirchhoff's Laws.  The proper
links are given here:

Expt 1 Expt 2 Expt 3 Expt 4 Expt 5