Well,
I found all I really wanted was some form of user log-in. I didnt need all the features offered by ACS, I did play around for a bit with auth-proxy, using freeRADIUS. FreeRADIUS is a great open source RADIUS server, but is definetly a little more difficult to setup. I switched over from XP to linux, in or to do so. Thus there was more work involved. eventually I ended up getting auth-proxy/FreeRADIUS to work for me, supplying a basic user login to access the network. although I eventually found auth-proxy to be a bit limited with what i could do with it. The problem I faced was that since the cisco was doing my routing, I needed to find somthing to provide a "captive portal" and do it in bridge mode so I could just stick it between my cisco and my network. I ended up going with Zeroshell, an open source router/firewall/etc which has a captive portal amongst other things you may find handy. I installed it an a ALIX board, and I'm running it in bridge mode, using just the captive portal. Like I said, this is just so I can have a basic user portal for network access, any deeper authentication services, such as device authentication/auditing, etc. this may not be able to help you. As far as I could tell, there really wasnt an alterntive for ACS to use. Take a look at Zeroshell, it may be helpful, it does quite a bit more than I'm using. it's still somewhat in development, but it's active development, and it seems to be on the road to being quite usefull.
hope this helps.
If you just need a basic RADIUS services, check out freeradius, I think it's pretty tough to get running on windows if thats your platform. if you run windows, I found clearbox radius server to be quite good, ease of use, and pretty well priced.
