It is generally arranged by teh RCL but there is an index to search for any particular Scripture passage. Not all the articles are helpful and some links are dead now but it does tend to be one of my go-to sites.
Working Preacher is a very helpful commentary site. It follows the lectionary but also includes a search index so that if there's a particular passage you want to look up regardless of whether it's on for this week or not you can.
A good commentary that I've often used when preparing for sermons (along with other sources) is the NIV Application Commentary. I don't know if that one's online or not though. What I like aboit it is that it not only offers text interpretation but also ideas on applying passages to every day living.