Try Rodi
all relevant links here
http://larytet.sourceforge.net/tryRodi.shtml
500K of binary if you run GUI and ~300K without GUI
0% CPU all the time, under 30M of RAM - usually in 25-30M range
multisource download is supported
the network is searchable and does not require any servers/super nodes/etc
to search the network you need only port number of the peer(s), for example 31211 and IP subnet, for example 192.168.0.0, no permanent IPs are required
client is implemented in Java and tested both on Windows and Linux
number of peers in the network is not limited (scalability of the current code was tested for ~1000 peers)
supports both standalone and applet mode - yes you can run it directly from any HTTP server without installing anything. you sit in your Univesity lab, you have a couple of minutes, you go to SF.net, you run 300K Java applet (CLI engine), a couple of commands and you are connected to your PC at home or any PC in the network.
feel free to ask questions on the discussion boards linked above
P.S. and i forgot the most important thing - you do not need any forums and discussion boards to use Rodi network, you don't need registration, you dont' have to worry about somebody loggin your IP address when you post your public key or PGP signature. Rodi is free as in Freedom open source project and available for everybody from SourceForge servers.