When I run Digsby it takes up 140MB of memory. When I run MS-Messenger it takes 50MB of memory. Is there a multipurpose chat app which takes < 10MB Memory. This is a big pet peeve of mine.
Miranda IM is probably the multi-protocol IM client with the lowest resource usage.
Pidgin is a bit 'heavier' but still far from WLM or Digsby (and probably more user-friendly than Miranda).
Both programs are open-source freeware. portable versions are available. however, they do have their limitations when it comes to video/voice chat support or file transfers.
I'm using portable versions of both programs; Miranda IM tallies about 8 MB memory usage, Pidgin 38 MB (connecting to 6 different protocols, that is - this includes about 5 MB for the PortableApps launcher).
I'm running Pidgin with 6 accounts active at once and 30 plugins enabled, and it's currently showing ~28M in the Windows Task Manager "Processes" tab. That seems pretty reasonable to me.