Teoria Music Tutorial:
Beats and Measures Link.
Time Signature Link.
Note Value Link.
Dotted notes and the Ties Link.
Rests Link.
Notes value shorter than a Beat Link.
Eighth and Sixteenth Notes Link.
Eighth and Sixteenth Rests Link.
Rhythm Dictation Link
MusicTheory.Net:
Note Duration Link.
Measure and Time Signature Link.