One of my favorites: Cells Alive!
www.cellsalive.com

This is a little advanced, but good for your upper level kids... or you could let them look at a book while doing this! This is an online labeling of one plant and one animal cell)
http://www.nmsu.edu/~molbio/cellgame/cellpin.html
http://www.nmsu.edu/~molbio/cellgame/CellGamePlant.html
 

Celebrate Cells Activity:
http://can-do.com/uci/ssi2001/cells.html 

 

Activities Helping Students to understand WHY cells divide: (Surface to volume ratios)

Good hands on foldable cube:  http://www.accessexcellence.org/AE/AEC/AEF/1996/deaver_cell.html

Good Explanation:

http://old.jccc.net/~pdecell/cells/cellsize.html

Using agar to show ratio/surface area:

http://www.iit.edu/~smile/bi9226.html

http://www.mrshelley.com/Honors%20Biology/surface%20area%20to%20volume%20of%20cell%20lab.htm

Another good foldable:

http://www.geocities.com/CapeCanaveral/Hall/1410/lab-B-24.html

Great Math Crossover for your advanced students!

http://www.ugrad.math.ubc.ca/coursedoc/math100/notes/zoo/cell.html

****Surface to volume ratio (of cube)... is interactive!!!*****

http://standards.nctm.org/document/eexamples/chap6/6.3/part2.htm

Mitosis and Meiosis Online Labs/Explanations

Mitosis and Meiosis identification lab online:  (Excellent photos with a quiz too!)

http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division.html

Cell Cycle Game:

http://www.nobel.se/medicine/educational/2001/cellcycle.html

Mitosis and Meiosis animations:

http://www.lewport.wnyric.org/jwanamaker/animations.htm

Meiosis animation:  (Needs macromedia shockwave)

http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/meiosis.html

Cell Cycle tutorial from Cells Alive!

http://www.cellsalive.com/cell_cycle.htm

Biology in Motion Cell Division: (Allows students to PRACTICE cell division!)

http://biologyinmotion.com/cell_division/

PBS Nova Flash Animation of Cell Division:

http://www.pbs.org/wgbh/nova/baby/divi_flash.html

Interactive cell division where you can name the number of chromosomes used:

http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division/CDCK/cdck.html

Flash player of mitosis and meiosis:

http://www3.telus.net/byngscience/sc10/Sc10~Biology/s_tan_division.swf