Streak is an amazing, flexible (and free!) app that can be added to Gmail or Google Apps email accounts. The idea is that it lets you set up different “pipelines” (for example, a sales pipeline, or you can also set up one for tracking guest posting pitches) and then it gives you a pretty, easy-to-use interface that sorts the emails associated with particular “boxes” (with the client funnel example, a box would be a potential client name).
Honestly, at least as far as freelance usage goes, Streak pretty much takes the cake for a lightweight CRM that makes it super-easy to keep on top of potential client follow up. It’s better than anything else I’ve found to keep you organized in getting from “just pitched” to “sealing the deal.” I highly recommend it if you use Google Apps or Gmail, although I’d also still recommend using and maintaining a separate system (like Contactually) for keeping track of and following up with past clients or customers of your products/classes. (The Client Follow Up Action Kit can also help with that!)