CompTIA provides vendor neutral certifications in Computer hardware, operating systems, Internet technologies, Internet security, and others. The certifications offered by CompTIA are recognized by leading computer and IT industries in the world.  The certifications offered by CompTIA are valid for life, in that they do not have an expiration date. However, CompTIA recommeds that the candidate upgrade the certification as new revisions are introduced, to stay current with the technologies.
Some of the most popular certifications offered by CompTIA are:
A+ Certification:
      This certification is aimed at testing one's knowledge in computer hardware and Windows operating systems (Particularly client side). It consists of two individual exams: A+ core hardware exam, and A+ Operating Systems exam. As the names suggest, the former covers the computer hardware including the BIOS start-up sequence, and the latter covers Operating Systems, primarily Windows Operating Systems. Linux is excluded from this exam, as it is covered in the Linux+ certification.  More details on CompTIA A+ certification.
Network+ Certification:
      This certification is geared towards the understanding of TCP/IP networking fundamentals.
Linux+ Certification:
       The CompTIA Linux+ certification tests competency for technicians with six-months experience installing, operating and maintaining Linux operating systems.
Server+ Certification:
      Server+ certification is aimed at testing mid- to upper-level technicians responsible for server hardware functionality.The  credential validates advanced-level technical competency of server issues and technology, including installation, configuration, upgrading, maintenance, troubleshooting and disaster recovery.
i-Net+ Certification:
      The CompTIA i-Net+ certification tests foundation-level competency in knowledge of Internet, Intranet & Extranet technologies.The i-Net+ certification is recognized as a baseline technical knowledge specifically designed to certify entry-level Internet and e-commerce technical professionals. i-Net+ certified candidates demonstrate knowledge and competency in Internet basics and clients,development, networking,
Internet security and business concepts.
Checkout CompTIA certifications here. The practice tests are available from simulationexams.com's comptia certification tests. Looking for an on-line version of practice tests, check a+ online tests