Uninstalling Software: The Basics
Most OS X applications are completely self-contained 'packages' that can be uninstalled by simply dragging the application to the Trash. Most applications create preference files which are stored in the /Home/Library/Preferences/ folder. Although they do nothing once you delete the associated application, they do take up some disk space. If you want you can located them in the above location and delete them, too.
Some applications may install an uninstaller program that can be used to remove the application. In some cases the uninstaller may be part of the application's installer, and is invoked by clicking on a Customize button that will appear during the install process.
Some applications may install components in the /Home/Library/Applications Support/ folder. You can also check there to see if the application has created a folder. You can also delete the folder that's in the Applications Support folder. Again, they don't do anything but take up disk space once the application is trashed.
Some applications may install a startupitem or a Log In item. Startupitems are usually installed in the /Library/StartupItems/ folder and less often in the /Home/Library/StartupItems/ folder. Log In Items are set in the Accounts preferences. Open System Preferences, click on the Accounts icon, then click on the LogIn Items tab. Locate the item in the list for the application you want to remove and click on the '-' button to delete it from the list.
If an application installs any other files the best way to track them down is to do a Finder search using the application name or the developer name as the search term.
There are also several shareware utilities that can uninstall applications:

AppZapper
CleanApp
Yank
SuperPop
Uninstaller
Spring Cleaning
Look for them at VersionTracker or MacUpdate.

For more information visit The XLab FAQs and read the FAQ on removing software.
Kappy's Personal Suggestions for OS X Maintenance
For disk repairs use Disk Utility. For situations DU cannot handle the best third-party utilities are: Disk Warrior; DW only fixes problems with the disk directory, but most disk problems are caused by directory corruption; Disk Warrior 4.x is now Intel Mac compatible. TechTool Pro provides additional repair options including file repair and recovery, system diagnostics, and disk defragmentation. TechTool Pro 4.5.1 or higher are Intel Mac compatible; Drive Genius is similar to TechTool Pro in terms of the various repair services provided. Versions 1.5.1 or later are Intel Mac compatible.
OS X performs certain maintenance functions that are scheduled to occur on a daily, weekly, or monthly period. The maintenance scripts run in the early AM only if the computer is turned on 24/7 (no sleep.) If this isn't the case, then an excellent solution is to download and install a shareware utility such as Macaroni, JAW PseudoAnacron, or Anacron that will automate the maintenance activity regardless of whether the computer is turned off or asleep. Dependence upon third-party utilities to run the periodic maintenance scripts had been significantly reduced in Tiger and Leopard.
OS X automatically defrags files less than 20 MBs in size, so unless you have a disk full of very large files there's little need for defragmenting the hard drive. As for virus protection there are few if any such animals affecting OS X. You can protect the computer easily using the freeware Open Source virus protection software ClamXAV. Personally I would avoid most commercial anti-virus software because of their potential for causing problems.
I would also recommend downloading the shareware utility TinkerTool System that you can use for periodic maintenance such as removing old logfiles and archives, clearing caches, etc.
For emergency repairs install the freeware utility Applejack. If you cannot start up in OS X, you may be able to start in single-user mode from which you can run Applejack to do a whole set of repair and maintenance routines from the commandline. Note that presently AppleJack is not compatible with Leopard.
When you install any new system software or updates be sure to repair the hard drive and permissions beforehand. I also recommend booting into safe mode before doing system software updates.
Get an external Firewire drive at least equal in size to the internal hard drive and make (and maintain) a bootable clone/backup. You can make a bootable clone using the Restore option of Disk Utility. You can also make and maintain clones with good backup software. My personal recommendations are (order is not significant):
1. Retrospect Desktop (Commercial - not yet universal binary)
2. Synchronize! Pro X (Commercial)
3. Synk (Backup, Standard, or Pro)
4. Deja Vu (Shareware)
5. Carbon Copy Cloner (Donationware)
6. SuperDuper! (Commercial)
7. Intego Personal Backup (Commercial)
8. Data Backup (Commercial)
The following utilities can also be used for backup, but cannot create bootable clones:
1. Backup (requires a .Mac account with Apple both to get the software and to use it.)
2. Toast
3. Impression
4. arRSync
Apple's Backup is a full backup tool capable of also backing up across multiple media such as CD/DVD. However, it cannot create bootable backups. It is primarily an 'archiving' utility as are the other two.
Impression and Toast are disk image based backups, only. Particularly useful if you need to backup to CD/DVD across multiple media.
Visit The XLab FAQs and read the FAQs on maintenance, optimization, virus protection, and backup and restore.
Additional suggestions will be found in Mac Maintenance Quick Assist.
Referenced software can be found at www.versiontracker.com and www.macupdate.com.