K.C.
These were recommended last year for students going into first grade, but they probably have activities for a variety of ages:
ADDITION
www.sheppardsoftware.com/mathgames/earlymath/bugabalooSho...
www.hbschool.com/activity/adding_bricks_k/
www.dositey.com/addsub/addex1.htm
www.fun4thebrain.com/addition.html
SUBTRACTION
www.hbschool.com/activity/day_at_the_beach_k/
www.ictgames.com/save_the_whale_v4.htm1
www.hbschool.com/activity/count_back_numberline/
www.fun4thebrain.com/subtraction.html
ADDITION and SUBTRACTION
www.mhschool.com/math/2003/student/factdash/popup.html
www.missmaggie.org/scholastic/dilemma_eng_launcher.htm1
PLACE VALUE
www.ictgames.com/sharknumbers_v2.html
MONEY
www.hbschool.com/activity/money_build_robot/
TIME
www.hbschool.com/activity/telling_time_grk