If you're a veteran cake-decorating-tutorial googler, you probably already know about Wilton Cakes. But if you're just beginning to use the web as a cake decorating resource, FYI, Wilton Cakes has THE treasure trove of pro cake decorating tutorials.
There are TONS here (1000 in Designs and Ideas alone). Wilton Cakes is a retailer, so they often tie the instruction to specific products. However, the HowTo information is generally good (& free) - regardless of what brand product you decide to use.