Why do I ask? Well, some of the best cross-border co-operation programmes are in the Nordic countries, and they also have some of the best websites as well - mainly in the local language, of course. So, if you can read Scandic, I can heartily recommend http://www.interreg.no/, which is the national INTERREG site in Norway. Yes, I know Norway is not in the EU, but they are hugely enthusiastic about all things INTERREG (more than some Member States if truth be told).
Another top site is http://www.interreg-sverige-norge.com/ for the Sweden-Norway programme. Both sites are well-designed, clearly laid-out, and contain lots of information for potential project partners. Admittedly, the lack of English information makes them less interesting outside the North to some extent, but they do demonstrate a positive use of the Internet that other programmes should aspire to.
There are many good, interesting sites out there - too many to mention in one blog entry (www.ctc.ee is another I have just discovered). Has anyone any favourite co-operation websites they would like to share? Let me know in the comments section.