I have a scenario where i have to establish peer-to-peer preshared key based VPN to a vendor. This vendor uses Cisco EasyVPN with Cisco VPN 3000 as access concentrator. I check the compatibility of the type of VPN from http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6635/ps7180/prod_brochure0900aecd80582078.pdf and it appears that it would only work on cisco hardware.
I am looking for a less inexpensive and possibly and open replacement to this to save my cost and also recommend to them. I have tried OpenVPN and thinking about vyatta.
They claim they can do whatever cisco does so ...
http://www.vyatta.com/
Assuming Cisco's EasyVPN is using some flavor of IPSec with pre-shared keys, you can likely use any SMB firewall appliance on your end, or go open source with pfSense and put it on your own hardware. On their end, you may not have any choice and a VPN Concentrator is likely in place for a reason: they have a lot of peers connecting to their endpoint.
Cisco Easy VPN appears to be using IPSEC - if it conforming to standard you should be able to get pretty much any router that supports IPSEC to connect to it. pfSense is brilliant and free, and should do the job.
There are lots of questions here on pfSense and it is normally very highly rated by people that are familiar with/use it.
I used to look after a pfSense box that was heavily used for site to site vpn's and the only downtimes ever were due to a few stupid mistakes by myself. If only I had been allowed to set up a 2nd box for failover......
Oops, no idea how I ended up on a question so old thinking it was current!
If you need to connect to a concentrator as-if using a desktop VPN client, "vpnc - client for cisco vpn concentrator" is your friend. I've used it with huge success. (http://www.unix-ag.uni-kl.de/~massar/vpnc/)
Now if you're connecting site-to-site IPSec, I'd recommend using something like KAME IPSec for Linux. It's quite complicated to setup, but we've managed to connect to almost anything with it.
We also use OpenVPN, but then, you'd need to convince the other party involved to run a OpenVPN server or client.