Growl

Growl is a notification tool that puts little unobtrusive message bubbles on your screen to let you know what's happening in the background.  It doesn’t work with many of Apple’s built-in apps, but does work with a number of third-party programs and extensions. These include file transfer programs like Transmit, instant messaging programs like Adium and Yahoo, dashboard widgets like eBay watcher, and many Mac twitter apps. This can be when an upload or download is complete (Transmit), when a new IM message comes in (Adium), or when your plugged-in status on your laptop changes (Unplugged).

1Password

Now available for the Mac and Windows, 1Password from Agile Web Solutions has earned a space on my drive. While I still use Little Secrets for a lot of miscellaneous information, 1Password shines in its ability to interface with multiple browsers and provide you with a convenient menu of applicable logins for the vast majority of sites including google, yahoo, logmein, and many banking sites. One click, or a command-\ and you're logged in.

Mail Act-On and MailTags

A pair of extensions for Apple's built in Mail application from InDev software. Act-On allows you to use keyboard shortcuts to redirect your current or selected mail via a set of custom rules. In my case I use it to file away several common general categories of mail, flag them or not based on whether I need to see it again (with a smart folder to view flagged mail), and forward them if needed, all in one easy step.

MailTags stands alone - allowing you to tag your mail much like Google Mail, letting you search for (or create smart folders to find)  mail based on key words that you specify, including useful presets like "Waiting" (for reply). It also allows you to append notes to your mail messages.

Of the two, Act-On is the most useful, and a strong recommendation.