For no other reason than *because it's there*, here is the result of asking Mathematica to factor RSA 100.

In 1991,

... the factorization took a few days using the multiple-polynomial quadratic sieve algorithm on a MasPar parallel computer.

and now,

It takes four hours to repeat this factorization using the program Msieve on a 2200 MHz Athlon 64 processor.

In comparison, Mathematica took little more than an hour. Unfortunately, it was not able to parallelize it (for a *real* comparison, the Msieve code would have to be ported over).

{% img center http://farm6.staticflickr.com/5510/12309071334*575cda01f9*z_d.jpg %}

**Update:** This has been much better documented on this StackExchange thread, which also links to notes on internal implementation, in case you're interested:

FactorInteger switches between trial division, Pollard , Pollard rho, elliptic curve, and quadratic sieve algorithms.

Finally, more on RSA Numbers, as well as a Mathematica Notebook, here