For IDS, I highly recommend Snort. We are running it at our college right now. There are others, I just haven't played with them a whole lot. For IPS, we are currently looking at Tippingpoint Technologies. We have also talked to StillSecure, ISS (which they never bothered to return my phone calls, go figure), and have looked briefly at LucidSecurity.
Understand that IDS is _only_ detection, it does not stop attacks. IPS stops attacks and does logging for forensics. I have some resources for information if you want them.
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I am Comptia A+ Certified