To learn about why you might want to get more Vitamin D3 in your life, I recommend starting with The Vitamin D Council's site. Their October 2008 newsletter inspired me to post this.
One thing I like about Vitamin D3 is that it's cheap, especially in the summer time when it's free (your skin makes it for you when it is exposed to sunlight)! When it isn't free, I buy it from iHerb.com whom I've used for many years with no complaints. If anyone is interested, feel free to use coupon code PEN753 for $5 off your order for first time customers. Disclaimer: If you use that coupon, I get a referral fee.
In case you were curious (and I was), the cholecalciferol in commercial supplements is probably derived from lanolin.
And here is a link to the specific brand of Vitamin D3 that I use:
While I'm mentioning it, here's the other supplement I take 400 - 500 mg of nightly:
Those magnesium people need to get their act together and form a council so I'd have something to link to. In lieu of that, here's something by Dr. Michael Eades on his favorite supplement.