1. MainMenu. Free. Superlative. Creates a neat little menu item on the top bar. Better than most other tools I have tried for this purpose, especially in its clean interface. Sometimes, if you have the pleasure of experiencing a situation when the Trash won't clean because OSX says that the "Application is still in use" but you're sure you quit it and it's not live anyway, MainMenu's "Force Empty Trash" is a fabulous tool to have at your fingertips.
   
2. Little Snitch: Tells you everytime some program on your machine wants to "call home" and connect to some server. Great flexibility in allowing the program to connect to a server, a port, or in general. Allow (or Deny) it to connect only once, or until the application quits, or Forever.
   
3. RCDefaultApp: Just as it is on any OS from Windows to Ubuntu, it often happens that you would like to associate certain file types with certain applications. On Mac OSX, we do have the same right-click contextual menu as Windows that allows "Open with [Application]" and "Make this the default application", but for some reason this doesn't always work, and occasionally doesn't even show up as an option. No matter. RCDefaultApp is the application that allows you to do that superbly, and then some.
   
4. Butler: Another small utility with a negligible footprint that allows for some nifty shortcuts to stuff already on your machine.
   
5. Perian: No Mac should be without this. This pretty much explains itself. There's a nice video tutorial here that shows how easy it is to install and then forget it. Suddenly your Quicktime (and iTunes) will be able to play a whole raft of video formats. If you want to be really equipped, get the Divx codec, the 3ivx, and Flip4Mac which plays wmv (Windows Media Player) files on your Mac. Of course if you get really frustrated there's always the tried and tested VLC Player.
6. CleanApp: The best application uninstaller out there, hands down. Don't believe for a minute when the OSX manuals tell you that on a Mac all you need to do is drag the application into the Trash and you're done. BS. Many applications (think Adobe) install several things in several locations. CleanApp 3 tells you all the associated trappings of these applications and allows you to uninstall them all together.CleanApp is not free, alas (there is always a poor man's AppCleaner, which does some basic stuff) but it allows for much more granular control. The best part: CleanApp has a "Logging" service that keeps track of whatever you install, and then knows in granular detail everything that you need to uninstall later; you can enable and disable this logging service at will, so it is useful to keep it generally off and only switch it on before you are undertaking a serious install of software, such as Final Cut Pro from Apple for example.
7. TinkerTools: To modify the many system preferences in your OSX that should have been made tinker-able but are not. Us Windows switchers are used to modding everything, so this is a fabulous tool.
8. Transmission: The best torrent client for OSX. Very simple, no-nonsense, and yet pretty interface. BitRocket is all google-juiced as it has been around longer, but it went down more often than Paris Hilton's pants.  Limewire now has an OSX version too, but I am done with crashing and slow download speeds unless you cough up a few dollars.
   
9. Candy Bar: If you really, really want to modify your icons. Panic is one of the better software developers for the OSX platform. Their Unison tool, a native OSX Usenet client is pure code poetry. There's a lot of iconography available at their partner website IconFactory. CandyBar is not free though. If you are short on cash, you can always try the somewhat barebones LiteIcon.
10. Vienna: The best and most elegant RSS reader client for OSX at the moment. Now if only they could sync it with Google Reader, Bob might be my uncle. How long has the Google API been out now!?
11. Chicken of the VNC: The best VNC client out there, connects without problems to Windows VNC servers too.
    

    

12. OmniDiskSweeper: As you start using your OSX, and installing applications and such, your hard disk usage keeps mounting (no pun intended). The fast, small footprint OmniDiskSweeper does this job faster than anything else on the market, including the somewhat visually prettier WhatSize.
    
13. Monolingual: Like Windows, OSX also comes with about a gazillion languages preinstalled, which take several gigabytes on your hard disk. Likewise, OSX the operating system also comes with a number of architectures such as PowerPC even if you have an Intel system, because the same OS needs to support older Apple hardware. Anyone who has bought a new system with Intel's architectures (the latest Macbooks or iMacs) can safely get rid of the other architectures. Monolingual is a simple, free utility that does exactly that.
    
14. Tech Tools Pro: Explanation coming soon.

A calendar on the desktop is a handy feature. My option to date has been to settle for Webshots, which is a nifty screensaver rotator utility and all that, but a bit bulky. Even when I turned off its screensaver functionality, its startup utility took up more time than I'm comfortable with.

Enter Rainlendar, the cute windows desktop calendar! Tricky names aside, this is simply a fabulous find. It's small, cute, has a negligible memory footprint, and looks great with its avant garde font.

It's a breeze to configure. You can place it anywhere on the desktop screen by simply dragging it and it latches into place. Best of all, you can have your To-Do lists and Events show up on the desktop as well. In a cool, semi-transparent manner (and the colors readjust automatically to make sure the text is visible.)

Best feature: It integrates right into my Microsoft Outlook through a MAPI interface — Calendars & Tasks. It even recognized my Archive.pst and pulled the calendar entries from there! No need to have a separate to-do list.

Brilliant!