JapanToDoor (www.japantodoor.com):
Rather expensive, they have three different account types with different service fee systems depending on your needs, but they do offer a very complete service, from online shopping, auction bidding and providing an address in Japan to send mail to. But, they are rather expensive... And one downside to the provided address in Japan, you have to inform them within 2 weeks in advance, that a letter or package will arrive at your address, or they won't accept it... Read their website for all service fee systems.
Ideal for: online shopping (without Japanese knowledge), auction bidding