LW 5 doesn't have a simple Chat function like LW 4 has. LW 5 users can share files & chat with friends who use gmail, LiveJournal or Jabber. More info here: Friends - LimeWire

LW 4 has a chat feature whereby you can try to chat with anybody who had chat enabled in LW's options. It's always been a bit unreliable but occasionally you could end up chatting to someone If you were to revert back to a version of LW 4 then of course you'd only be able to chat with other LW 4 users. If you want to try that then check out these threads for more info & links...

http://www.gnutellaforums.com/340213-post427.html

Downgrading to LimeWire 4 from 5 & resuming incomplete downloads
How to use Bitzi Web Lookup .. & How to use Magnet Links & Direct Connect/Browse/Chat

FrostWire has a chat feature (might be a bit hit & miss like LW 4) & also IRC chat.
FrostWire.com - Official Website - Download Free P2P BitTorrent/Gnutella Client - No Subscriptions, Just Download and Install

Other than that, I'm not sure which other Gnutella clients have a chat feature. Maybe use Google to search for a bit more info