Build Your Sys/Net Admin Cert Collection
Generally, some combination of vendor-neutral and vendor-specific credentials makes for a compelling personal portfolio when seeking work, or for those trying to enhance their technical and professional credibility to climb a career ladder. It's also typical that lower-level certifications are often vendor-neutral, whereas more advanced ones are more likely to be vendor-specific (except for deep technical specialties such as information security, protocol analysis, and so forth). By using the marketplace as a yardstick to measure the worth of the credentials that interest you (which means answering questions such as "How often does it appear in job postings? Classified ads? How does it rank in salary surveys? What's the buzz on the cert?" and other, similar queries to get a sense of what's hot and what's not), you can put together a good collection of credentials to fit your technical interests and job aspirations.
That said, the most popular vendor-specific credentials (ranked by size of certified populations) come from Microsoft, Cisco, Novell, and Red Hat. On the vendor-neutral side, the biggest numbers fall into the CompTIA and LPI camps.