Good open-source, multiplatform, secure IM client?

Phil O. writes: "I work for a company with 30+ locations across North America. Some offices have hundreds of employees; some only a dozen. We're looking for a multi-platform (Mac and Windows for now), secure IM client that we could implement across the organization. One group is pushing for Microsoft's solution, but it has a number of drawbacks (including cost). What other options are out there, and what has worked well in similar situations? A big concern for the company is security. Thanks."