One of my favorite chords is the add9 chord. You can create it by taking your favorite major chord and adding the note that is one whole-step (i.e. two frets) above the root. For a C major chord, the note two frets above C is a D, so our Cadd9 chord contains:
C · E · G · D
The D is the 9th of the chord and hence why it's called an "add9" chord. You take the D and you stick it on top of your major chord voicing.
One of my favorite voicings for a Cadd9 chord is shown below:
Note that this voicing is almost identical to your standard open-string C chord, but with a D on the 2nd string instead of the C. Sometimes I include the top E string when I play this chord, and sometimes I don't, but both versions sound great. Listen to the example below:
