These are tools I use frequently, but also look to for product inspiration.
Mailchimp: Mailchimp stole my heart years ago. Their analytics dashboard and subject line researcher have put them lightyears ahead of any other email marketing platform.
Basecamp: great for managing design assets. Hate that I can’t preview documents before downloading them.
Highrise: the only CRM worth using.
Metamarkets: an incredibly powerful tool for large scale ad campaign management.
Optimizely: A/B test like a boss.
Uservoice: Stay ahead of your backlog. Populate uservoice feedback with a few items you think are essential for the next phase of your site and get feedback from your actual users. Users can also add their requests here too.
Google Analytics: More data than most people know what to do with
Pivotal Tracker: The dead simple way to track development progress.
Keynote: Second best to white boarding. Keynotopia Bundles for snazzy mobile and iOS interfaces.
Learning to love:
Moqups - still not sure how I feel about Balsamiq