http://www.slexchange.com/

Like SL Boutique, no list of Second Life shopping sites would be complete without SL Exchange! Shop and buy on the web (wonderful to do when the grid is down!), then get delivery in-world (the grid must be up and you must be logged into SL to take delivery of an item.)

Also handy are the product reviews. They can be a real help. The product descriptions are created by the individual seller/creators, though, and sometimes the product descriptions can be woefully short of details and real information about the product. However, some items have great descriptions and details.

A great variety of stuff to buy is available on SL Exchange, with more coming online all the time. The only caveat I can think of is that sometimes the creators will sell certain items for less in-world - they want you to visit their in-world stores. So if you are buying something significant, you might want to check it out in an in-world store as well before buying it.