IEEE Distributed Systems Online. The first free IEEE journal. Topics covered include:

# Cluster Computing
# Collaborative Computing
# Dependable Systems
# Distributed Agents
# Distributed Databases
# Distributed Multimedia
# Grid Computing
# Middleware
# Mobile & Pervasive
# Operating Systems
# Parallel Processing
# Real-Time & Embedded
# Security
# Wide Area Systems

Each topic area also maintains a page with links to new publications, journal and event news in that area, etc. Very interesting.