Rhythm Exercises
http://www.izap.com/~pinnacle/rhythms/index.html
An introduction to advanced rhythmic groupings can be found here. The site includes notation examples, batch files for listening, and annotations on how they can be approached with different instruments.

Theory on the Web
http://www.smu.edu/totw/meter.htm
Clear explanations of simple and compound meters are offered at this site, with audio examples.

Polyrhythm Page
http://www.xs4all.nl/~marcz/Polyrhythm.html
This site provides an introduction to polyrhythmic patterns, with definitions and notation examples.

Rhythm Web
http://www.rhythmweb.com/
Links to musical cultures from around the world, information on percussion instruments, and on line tutorials are included in this site, with audio examples.

Rhythm and meter
http://library.thinkquest.org/16020/data/eng/text/education/theory/rythm.html

Meter basics
http://www.music.vt.edu/musicdictionary/appendix/meter/Meterbasics.html

Syncopation
http://www.mmissary.com/theory/theory35.jpg

Tempo
http://www.music.vt.edu/musicdictionary/appendix/tempo/tempo1.html