Hi there! I'm one of the Evolve devs (mamu on Evolve)--just saw this thread show up in our referral logs and thought I'd chime in a bit. Feel free to ask more questions and I'll reply whenever I have time.
I see one of the things Nnexxus mentioned upthread was our party system. This might be one of the more useful things for the GOG community to try out. It's basically a VPN, text chatroom, and VoIP channel all coupled together. Party leaders can choose who is allowed to access a given party. Parties can also be formed out of a matchmaking engine we've built. The VPN is fairly well tuned for games--it's peer-to-peer, uses actual private LAN IPs (in the 10.x.x.x range), doesn't have any size caps (beyond ~253 people, the size of a single subnet), and allows broadcast traffic to pass through (needed for many LAN-enabled games). We think it's a pretty good way to play multiplayer games that might not have master servers or working server browsers anymore (such as Dungeon Keeper, etc).
Gazoinks: It's a little bit annoying because even if you have notifications off, the Steam chat window still pops up (can you turn this off?) but it is indeed still nice. The addition of Facebook and GTalk is nice too, since I like to talk to my friends but don't want to have to keep unnecessary interweb tabs open.
EDIT: Also, it'd be nice if you could have it open on multiple computers at once. It doesn't host your games like Steam, so it doesn't seem like there's much reason not to.
Agreed that the Steam-shows-your-messages-too issue is rather annoying. It's a side effect of using Steam chat via Steamworks. Unfortunately, there are only two ways to interact with Steam chat: using the full Steam protocol or using Steamworks. The former doesn't have the double-message issue because it outright kicks you off of Steam (since Steam doesn't support multipresence as you pointed out). We're hoping we can eventually somehow suppress the Steam chat windows while using Steamworks, but we haven't looked in to it just yet. We're also intending to at some point support connecting to Steam chat via the full Steam protocol.
You're quite right that there's no reason you shouldn't be able to log in to Evolve via multiple computers. It's something of a poor design choice on our part from much earlier on in the development of the platform. We're intending to rectify it eventually... we're a two-man engineering team, so these things sometimes take time.
Gazoinks: Bump for question: Is there a way of manually adding games? For example, I like to use emulators for when I don't feel like bringing out the old consoles, but Evolve doesn't sense them. Can this be mended?
We're planning to support detecting games within emulators in the future--which is the only reason we haven't just added the emulators themselves. We actually do this for several of GOG's DOSBox-powered titles already, but the technique we're using for them doesn't really apply to things like SNES9X or PCSX2.