What should I use to scan for Trojans or Keyloggers?

Question

I've heard conflicting conversations about not using anti-virus or that OS X has built in protection, but I wanted to know what the best way to check for Trojans or Keyloggers is so I can give my computer a looking-over before backing it up.
I'm using OS X 10.6.8

Answer 1

Conversations?  There have been a few of those.
I'm not aware of a tool that's particularly reliable about detecting malware, nor one that might (for instance) catch hardware keyloggers.
The usual questions around your situation apply; I don't know how you use the system, nor what exposures or risks you might have or might face.  For instance, how sensitive is your information, who has had access, and what are the circumstances associated with the access, and what's been loaded onto the system? 
If you're incautious or blasé about your computing practices and fond of downloading from torrents or acquiring cracked software or downloads from sketchy sites, then the anti-malwaretools generally won't save you.
In general: just back it up, back it up regularly, and keep various copies of the backups offline or remote, or both.  Backups and caution are among the best available anti-malware tools.
If you're fond of installing random stuff and particularly of entering your administrative password when prompted — that password is the proverbial keys to the security kingdom, to your address book, etc — or if your login passwords have been compromised (or have kids around that install all sorts of Free Stuff! and Free Games!), then change all your passwords, and install Little Snitch or equivalent, or implement and use network perimeter monitoring.  These monitoring tools catch (unexpected) outbound network connections, such as that Facebook watering hole attack.
Better still: upgrade to 10.7 or 10.8 as your hardware permits, as 10.6 is rather old and lacking some of the newer capabilities here, such as Gatekeeper.  Disable the Java web start plug-in to disable Java (and don't install Java after you upgrade to 10.7 or 10.8), and remove Adobe Flash Player, and disable the "open safe attachments" setting.  And if you're (necessarily, or otherwise) paranoid, install Little Snitch or network perimeter monitoring.
If you feel obligated to use a malware scanner (the hit rates on the malware-scanning tools are far from great, and the implementations of most aren't that much different from the malware itself in terms of how the scanning software can insinuate itself into the system and can sometimes then trigger issues with stability and operations), then ClamAV is the usual recommendation around the forums.  OS X Server includes that, and there are various discussions around getting ClamAV going on OS X client.
Once your operating system software has been compromised, the usual path is a wipe and reinstall, or a wipe and install of a backup prior to the breach, followed by steps to prevent a reoccurance.  Decontamination of a system is difficult, at best.  If your hardware has been compromised, you're in deep sneakers. 
If you want to be or need to be paranoid due to the information involved or folks that have had unfettered access to your system hardware, then check for dongles or other unexpected external devices, and start replacing your equipment. The "better grade" keyloggers can be stored in USB devices, and can be mounted within your hardware.  The better-grade gear can be difficult to locate, short of disassembly.  Also check your local network, as it's feasible to monitor traffic there, whether within a modem, or within a device added to your network.
Again, how paranoid you wanr or need be here depends highly on what you've been up to, and who might be after you, and a whole host of other details...