Ed Helper has metric lessons here: http://www.edhelper.com/metricsystem.htm

All Math.com has a metric converter, sites to help you find worksheets and lessons and much more:
http://www.allmath.com/Worksheets.php

Math2 has a reference table for many types of math. It can be downloaded.
http://math2.org/

The Science Spot has some nice hands on lessons for measurement:
http://sciencespot.net/Pages/classmetric.html#Anchor-General-49575

Math League.com has a complete measurement reference guide:
http://www.mathleague.com/help/metric/metric.htm

The Math Enhancement Program has free math lessons for all ages.
http://www.cimt.plymouth.ac.uk/projects/mep/default.htm

 AAA Math has many lessons and more on measurement:
http://www.aaamath.com/mea.html#topic50

Academic Skill Builder has many free online math games: http://www.arcademicskillbuilders.com/games.htm

Make your own worksheets at: Superkids Math Worksheet Creator: http://www.superkids.com/aweb/tools/math/index.shtml

Homeschool Math also has a worksheet creator: http://www.homeschoolmath.net/worksheets/

Noetic Learning has some free math sheets: http://www.noetic-learning.com/mathdrill/