I've moved a large chunk of my userland experience off of the modern Linux box and onto the old Sun Ultra 1 running OpenBSD 4.7. My web browser, desktop session, BitTorrent client, and MP3 player are still running on the newer Linux box.
Email - alpine is the mail client of choice for me on this box. After a bit of reading and a bit more googling, it wasn't too hard to get this set up on my primary mailbox. I haven't yet gotten multiple mailboxes going but I've learned it is possible and probably not too hard to set up. It's working really well on the old Sun, and has a decently small footprint on the CPU.
IRC - I've been using irssi for many years. It was a simple matter to rsync my ~/.irssi directory over to the Sun box and fire irssi up. It, too, has a very small footprint on the CPU.
IM - This is where things get hairy. I've used finch before and have had mixed feelings about it. finch's behavior inside of a GNU screen session has left a bit to be desired in the past, mostly in terms of not gracefully resizing itself when I attach from different terminals or resize my terminal window. Under OpenBSD, trying to install finch requires X11 and a chunk of Gnome related libraries. Ugh. jbroome in Freenode/#trilug mentioned that I might like to try bitlbee with my existing irssi installation.
This dependency problem appears to be a recurring theme. I also ran into X11 dependencies trying to install horde. Whiskey Tango Foxtrot?!? horde tries grabbing php5-gd, which seems to require X11. I'm thinking this is probably a problem with how OpenBSD packages are being built.
Ultimately I relented and installed the X11 libs since I have plenty of disk space. If I had left the original 4G disk in place, I'd probably be more upset about this.
Speaking of disk, the 10MB/sec SCSI controller has been a bigger bottleneck for me than the 167MHz CPU or meager 128MB of RAM. Indeed, I still have 29MB of RAM free. But big disk operations seem to bring everything to a crawl.
To that end, I'm going to look into my options for a faster sbus SCSI adapter. Suggestions? I do have a bunch of Ultra160 and Ultra320 disks that I've picked up on eBay over the years, plus a StorEdge D1000 array.