I love virtual labs. I display them on the TV in my classroom to punctuate lectures, and assign make-up labs based on them. Some of my favorites are:

General

http://web.umr.edu/~genchem/links.html

http://www.latenitelabs.com/ (cost associated - try the FREE TRIAL PERIOD).

www.OldManHonda.com (Bonus vocabulary games)

http://www.hazelwood.k12.mo.us/~grichert/sciweb/applets.html (CD with worksheets to accompany these virtual labs available for a mimimal cost.)

http://web.umr.edu/~gbert/links.html (Tutorials and simulations for Chemistry--this site also contains a number of interactive tutorials on introductory and advanced chemistry topics).

Gas Laws:

http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm

http://zebu.uoregon.edu/nsf/piston.html

http://www.phy.ntnu.edu.tw/java/idealGas/idealGas.html

Titrations:

http://lrs.ed.uiuc.edu/students/mihyewon/chemlab_experiment.html

http://www.cc.edu/~jtowell/Titration/titrate.html

Photoelectric Effect:

http://www.usd.edu/phys/courses/phys431/notes/notes5g/photoelectric.html

Modern Atomic Theory:

http://www.colorado.edu/physics/2000/index.pl?Type=TOC

Organic:

http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi

http://atom.chem.wwu.edu/dept/vmolckit/molecule.html

Carnegie Mellon has a whole virtual lab that can be accessed on the web or
by getting their free software. See
http://ir.chem.cmu.edu/irproject/