Recently I have started actually using Gmail Labs features which are just awesome, if taken a little bit seriously for productivity. As a regular Google user, I find that I would like to integrate different Google services while not cluttering it too much. Of course iGoogle is first name which comes in mind but then there are people like me who use Google primarily either for search or email and want secondary things lurking around so that they can be accessed easily. iGoogle keeps all things boxed. You can integrate Gmail with iGoogle but to read even a single email, you will be directed to Gmail by iGoogle. That is where I find Gmail labs having upper hand.
Every widget which you select in Gmail Labs is integrated with Gmail. Plus there are lot of Gmail utilities like adding signatures after write-up than at the end of email, adding Mark as Read button etc. which definitely save your time and increase your productivity. Presently I am using seven such features provided by labs which include integrating Google Calendar and Google Docs.
Oh yes, forgot to mention that you don’t need to worry about Gmail getting heavily loaded because of adding these features. Just for sake of mentioning, Gmail is an app from Google where you can find one of the best AJAX implementations and it handles web-page so efficiently that the loading was never a concern.