I’ve been using this free CRM software service called Streak for Gmail at www.streak.com and it is awesome. Great for tracking leads or really any project between people. They have some built in projects for customer support, deals, etc but you can really set it up to do just about anything. It even includes free email tracking to see if your email is getting read. Highly recommended!
If you’re a small business that uses Gmail you’ll definitely want to give this a try to better organize the Gmail inbox.
UPDATE (1.30.17) Three years later and this company continues to improve the Gmail plugin. When I first started using it, they had just launched and now it has over 5000 reviews in the Chrome store with 4.5/5 rating. It still gets my highest recommendation.