if you're trying to get into a field in which you don't have 'formal' background, i.e. either work experience, degree or both.

In general:
\tCisco certs are very good. They require hands-on in addition to test-taking skills.

\tThe RHCE (Red Hat Certified Engineer) sounds pretty good as the emphasis is predominantly on demonstrated skills. (1 hr. multiple choice exam, six hours hands-on testing, I think)

I have an MCSE (I'm going to let it lapse, if it hasn't already), a CNE (NetWare 5) and a Solaris cert (Sys Admin 1). They'll get you in the door, usually.

Unfortunately, I'm not sure certs are as valuable now as they used to be, since it seems that employers (via their HR reps) have incredibly unrealistic checklists of 'required skills' for applicants. (You know, like "must have at least five years of C# background")

Personally, I feel certs don't *hurt* to have, especially if you can get someone else to pay for them.