The TOP500 project ranks and details the 500 most powerful publicly-known computer systems in the world. The project was started in 1993 and publishes an updated list of the supercomputers twice a year. The project aims to provide a reliable basis for tracking and detecting trends in high-performance computing and bases rankings on HPL, a portable implementation of the High-Performance LINPACK benchmark for distributed-memory computers.
The TOP500 list is compiled by Hans Meuer of the University of Mannheim, Germany, Jack Dongarra of the University of Tennessee, Knoxville, Erich Strohmaier and Horst Simon of NERSC/Lawrence Berkeley National Laboratory.
The list is updated twice a year. The first of these updates always coincides with the International Supercomputer Conference in June, the second one is presented in November at the IEEE Super Computer Conference in the USA.

Project History

IBM Blue Gene/L (since 2004.11)
NEC Earth Simulator (2002.06 - 2004.11)
IBM ASCI White (2000.11 - 2002.06)
Intel ASCI Red (1997.06 - 2000.11)
Hitachi CP-PACS (1996.11 - 1997.06)
Hitachi SR2201 (1996.06 - 1996.11)
Fujitsu Numerical Wind Tunnel (1994.11 - 1996.06)
Intel Paragon XP/S140 (1994.06 - 1994.11)
Fujitsu Numerical Wind Tunnel (1993.11 - 1994.06)
TMC CM-5 (1993.06 - 1993.11) The Systems Ranked #1 Since 1993
The following table gives the Top 10 positions of the 29th TOP500 List released during the International Supercomputing Conference (ISC07), June 26-29, 2007 in Dresden, Germany. http://www.supercomp.de/isc2007/

Current List (June 2007)
Taken from the official TOP500 site:

While the No. 1 system is still unchallenged, the rest of the TOP10 experienced large changes since November 2006.
The new and previous No. 1 is DOE's IBM BlueGene/L system, installed at DOE's Lawrence Livermore National Laboratory (LLNL) with a Linpack performance of 280.6 TFlop/s.
The upgraded Cray XT4/XT3 system at DOE's Oak Ridge National Laboratory is the third system ever recorded to exceed the 100 TFlop/s mark. It is No2 with 101.7 Tflop/s.
It ever so slightly edged out Sandia's Cray Red Storm system, which holds the No. 3 spot with 101.4 TFlop/s.
Two new BlueGene/L systems entered the TOP10. They are both located in the state of New York and represent the largest academic supercomputer installations.
The No. 5 system is installed at Stony Brook, NY at the New York Center for Computational Science (NYCCS) [4].
The No. 7 system at the Rensselaer Polytechnic at the Computational Center for Nanotechnology Innovations (CCNI), Troy, NY [5].
The new No.8 system was built by Dell and is installed at NCSA in Illinois. It was measured at 62.68 TFlop/s.
Just behind on No. 9 is the largest system in Europe, an IBM JS21 cluster installed at the Barcelona Supercomputing Center, with performance of 62.63 TFlop/s. It held the No 5 spot just 6 month ago.
The No. 10 was captured by a new SGI system installed in Germany at the Leibniz Computing Center in Munich. It was measured with 56.52 TFlop/s
The first Japanese system is at No. 14. It is a cluster integrated by NEC based on Sun Fire X4600 with Opteron processors, ClearSpeed accelerators and an InfiniBand interconnect, installed at the Tokyo Institute of Technology. Trends
The MDGRAPE-3 supercomputer reportedly reached a one Petaflop calculation speed, faster than any of those on the TOP500 list, though it does not qualify as a general-purpose supercomputer, and cannot run the benchmarking software used to gauge the speeds computed for the list.
Grid computing systems are also not included on the list.