Least obtrusive is possible Google Talk, but not as widely used as others.
Personally I use ICQ 2000, since it's less annoying than the later versions.
Note that there's also a webbased version of ICQ called ICQ2Go, that runs in a browser and does not require you to install any software on your PC. Great if you only expect to use it for a 'short' period, e.g. working on one project or something.
Linkage: http://www.icq.com/download/icq2go/
Personally I use ICQ 2000, since it's less annoying than the later versions.
Note that there's also a webbased version of ICQ called ICQ2Go, that runs in a browser and does not require you to install any software on your PC. Great if you only expect to use it for a 'short' period, e.g. working on one project or something.
Linkage: http://www.icq.com/download/icq2go/