Cisco: Capable, especially with the "external routing protocols" such as BGP. This is starting to change though. OpenBSD and FreeBSD make excellent routers and have _more_ functionality than a Cisco. Ciscos tend to be a bit under horsepowered unless you are running extremely high end routers. Access lists on a Cisco, when you start getting into the 45 MB/s range, are too costly. We are looking at replacing our Cisco with a Juniper (wire speed routing with access lists.)

OpenBSD: Very reliable. Simple and clean. The most secure OS (out of the box.) A great choice for any box that is sitting outside the firewall, or the firewall itself. Eventually a good router as well (depending on the interfaces available.) A swiss army knife of routing possiblities. Lags a bit behind FreeBSD on features. Restricted to single processor. Not the easiest thing to upgrade. Stick two drives in it and periodically dump and restore running disk to the backup.
The nice thing about using Intel based boxes with capable and free OSs is that you can have two of them. In the event of failure you can switch over. If you are upgrading, upgrade the one that is not currently active and then switch over. You have instant fallback capabilities.

FreeBSD: Very reliable. Not so simple or clean as OpenBSD. Secure, but not as agressively secure as OpenBSD. More features. Multi-processor. Great choice for a box running services such as Mail or Web. Upgrading is fairly easy, but still a little tricky.

SGI: Very reliable. We have had SGIs run for a whole year with 100 percent uptime. Very scaleable. Very nice to upgrade. Nice for mail, NFS services. Good for web, although there are more software choices for the BSDs. Quite attractive!

Linux: Very reliable. Don't like upgrading these at all. Number of features and pace of change can be scary. Things don't always feel well designed. Pieces are thrown out and redone, which can make transitions between versions a pain. Not as reliable as the BSDs. We only have one box now and that is to run some Linux specific software. We have tried it and moved on to the BSDs. Still, Linux just gets better and better over time.

MacOSX: - Darwin, the "core os" is very reliable. Nice to upgrade. Can run "normal people" type apps such as "Office." Has a FreeBSD "personality" sitting on top of the "Mach" kernel. Great for a workstation. Very easy to integrate Unix apps with GUI front end. This is what people use at their desks.