Activision is the latest publisher to offer some of its catalogue of titles on Valve's Steam service.

The first titles to be released on Steam are Call of Duty 2 ($39.95), Call of Duty ($19.95), Call of Duty: United Offensive ($19.95) and Neversoft's GUN ($19.95). Although the publisher has announced its plans, it hasn't specified a date when the games will be available.

In recent weeks Majesco has also joined Steam, offering BloodRayne 1 & 2, Advent Rising and the overlooked Psychonauts.