Look Into The Ones Below:
iSpyNow - www.exploreanywhere.com
PC Activity Monitor Pro - www.keylogger.org
remoteSpy - www.ispynow.com
Spector - www.spectorsoft.com
KeyCaptor - www.Keylogger-Software.com

I don't think he stated his requirements.

if w1mmy is interested in learning more about keyloggers and compiling his own, a couple of options are available on irongeeks site:

http://www.irongeek.com/i.php?page=security/keylogger

and on White Scorpions site:

http://www.white-scorpion.nl/programs/index.html

The source is available so these can be adapted for your own need or modified to work work with you favorite AV.

Syn

 Since the original poster wimm1y didn't really specify his requirements other than reliable or how he intends to use it, I am not sure how to advize. The ones listed by Kris would be fine if by reliable you meant actually produce results advertised. Heck, I have an old copy of Ghost Keylogger that I use on occasion that works great. It even sends copies of the log to my email and has been very reliable. The only problem with such apps is the can be caught by AVs or even some anti-spyware programs, which in turn can make all of them potentially "unreliable".  Thats why I wrote my own. Its not hard and if you are doing a pentest that involves going into an environment that is protected, you might  consider that approach.

SynJunkie above gave a link with some keyloggers you can compile. The first place to start is to grab the source code and review it. Get as much source code out there and get a general idea of how loggers are coded. Then from there you can start to make changes. The idea is to change the source code until the sig is no longer detected. Now what might happen is , depending on how much you have to rewrite it, you lose some functionality and possible stability.  It really depends on the quality of the code you are editing.  But whats left might be all you need to get you a password and not be detected.  This is the down and dirty way and is really more just "hacking up" the code. Advantages are its a fast way to do it and doesn't require a superior level of programming skill.  On the other hand , if you are great at programming, you will take the time to write on from ground up making sure to approach it from a different angle than whats available. 

I've just finished reading the 1337 h4x0r h4ndb00k and the final 'project' is a keylogger written in VB. Whilst I wouldn't recommend this book for anything serious but it is humuorous and this section may just give you the basics you need.

Happy hunting

Don't forget hardware keyloggers. You can even roll your own.

http://www.keelog.com/diy.html