SubEthaEdit has always worked well for LAN collaboration, but many users found the manual setup for sharing over the internet to be cumbersome. The Coding Monkeys answered the call with a couple of new ideas. First, automatic port forwarding handles establishing your channel outside of your LAN transparently, supporting all routers with UPnP or NAT-PMP capabilities. Second, iChat invites allow users to drag iChat buddies onto a document to automatically invite them to share in your coding ecstasy. This feature is Leopard-only, although I'm unsure if that applies to the invitee as well.
The iChat invites go a step further with "Friendcasting." You can view and connect with your friend's friends with the click of a button in the connection browser. You can disable this feature at any time.
If you're not familiar with SubEthaEdit, but happen to use Panic's Coda, then you're already familiar with the engine it runs on as it was licensed to Panic in 2007 for the web development package. If you're looking for a code editor and haven't tried it, it's definitely worth a shot, especially if you work in pairs or teams. TextMate and I have a monogamous relationship, but I'm allowed to look. Fair enough.
SubEthaEdit costs 29 euros (about $45 right now). Visit the SubEthaEdit page for a 30 day trial and a screencast of the new features.