Every second year for more than 50 years, the Particle Data Group has been compiling the best-of data about all the observed particles studied by particle physicists. The 2009 interim update was just released today and you can dig through all the gory details here.
It's not a resource for everybody, but if you want to see the level of detail that particle physicists need to consider when studying their field, it is worth a browse. You'll discover just how many different observations need to be made to settle on precise estimate of a particle's mass, interaction strength, or how often it decays into each type of daughter particle.
You could, for example, see the compilation of data on the mass of the Higgs boson (PDF). Or perhaps you'd rather check out the upper limits on the sizes of extra dimensions (PDF). Dark matter lives in the section on WIMPs and Other Particles (PDF).
Now, if you're really up-to-date, you'll recognize these numbers only include published data up until January 15, 2009. That's because you can't simply take an average of all the different experiments. The Particle Data Group digs through all the results, and combines them in a statistically valid way to achieve a best global result, which typically becomes the result used by particle physicists until the next update of the PDG. That takes a bit of time so there is a slight lag in publishing the compilation.
If numbers aren't quite your thing, and they're really probably not unless you are a particle physicist, you might find the review articles in the PDG more useful. They are only updated in even-numbered years, but they are a very useful summary of the current thinking about many topics in particle physics. They're still aimed at physicists and quite technical, but if you want to leap in, they are worth a look. The well-thumbed print copy of the PDG is a fixture on the symmetry editorial desk partly for the numbers but often for the review articles.
So next time you've lost a K4*(2045) and you want to know where to find it again--hint, look for the K*(892) and the three pions that are one tell-tale sign--or you just need to ship a few bazillion top quarks and want to calculate postage, the PDG will have all the information you could possible need.
