The following certifications have in common that you can self-study for the exams and do not need to attend expensive classroom training. Especially in unsure economic times these certifications can give you an advantage and improve your career.
The CCNA (Cisco Certified Network Associate) certificate is for everyone who needs to deal with networking. It is a well known, practically oriented certification that will help with finding a new job. The exam requires a lot of studying and you will need to learn about installing, configuring and managing network devices; about TCP/IP (including IPv6), subnetting, switching, routing, VLANs, NAT, Cisco IOS, wide are networks and network security. It helps to have hands on experience with Cisco hardware but you can also use a network simulator like RouterSim (costs money) or GNS3 (free).
To become a CCNA you can either take one longer exam (called 640-802) or pass two separate exams (ICND1 and ICND2). You can register for an exam via the Pearson VUE website.
PRINCE2 is a project management methodology and certification and as such of high importance for IT where you are facing projects daily, from setting up new users to launching a new product. It does not make you a project manager which requires years of practical experience but it gives you a better understand of the language, processes and structure of projects. If your goal is to actually become a project manager you might also want to look into PMI certifications like CAPM and PMP.
This certification from the Linux Professional Institute (LPI) tests your Linux knowledge, mainly in regard to the command line tools. The topics include software installation and configuration, working with command line tools, hardware configuration, user management and everything you need to run a server like webserver, database, DNS, mail, network and security fundamentals.
While you might need to learn some things that you will never use the certificate covers a broad range of topics and proves you are a skilled Linux administrator. It requires intensive learning, even if you have some years of experience with Linux.There are self study materials available for free online, e.g. at IBM developerWorks, but I would recommend you also get a book that contains some exercises and practice them. The exams ask very specific and sometimes confusing multiple choice questions for example on configuration files or Vim keyboard shortcuts that you might be able to easily answer if you had a computer in front of you but which are harder to answer in the test.
You need to pass two exams, LPI 101 and LPI 102, to receive the LPIC 1 certificate. Register first at lpi.org to receive your LPI identification number and then at a test center of either Pearson VUE or Promertic (both are very similar so it does not really matter which one you choose).
ITIL is a is a set of best practices for managing IT services. The ITIL foundations certification is therefore often mentioned together with ISO 20000, the international standard for IT service management. ITIL is a personal certification though while only companies can get an ISO certification.
You will learn about the concept of best practice, what a service is, roles and processes according to ITIL, lifecycles, continual service improvement. This is a bit more theoretical and boring than other certifications and it will not help you much if your company does not want to implement ITIL and / or ISO. But maybe you can be the one to introduce service management and show you have an understanding not only of IT but also of the business side. The exam is rather easy to pass, ITIL is about practices that have proven themselves helpful in companies and most of it is common sense.