Spike's free; its unique feature is that it uses Zeroconf networking to discover other Spike clipboards (aka Rendezvous, OpenTalk on Macs). In a LAN of spike users, you'll see all the other spike users' clipboards. Anyone can copy what you've copied. (You can password protect things, have clipboard you have to explicitly drop things in, etc, if you like). Very nice. If it did IM too it'd be killer.
The downsides: not as featureful as the other enhancers, and its a bit buggy (eg drag & drop is broken)