In the interest of competition, Sonoma Wire Works offers a similar service through their excellent RiffWorks software. The technology, called RiffLink, is built into RiffWorks. Although still in beta, I've heard second hand that it works pretty well.
The excellent Line 6 Guitar Port RiffTracker comes with a version of RiffWorks as well as a passel of Line 6's famous amp and effects models. In addition to hosting RiffLink, RiffWorks is an excellent computer recording and practicing platform developed especially for guitar and bass players.
Update: Apparently, RiffLink works really, really well because it's already inspired a band. I'm proud to present to you the RiffLink Junkies. Enjoy, people, enjoy.