ECM software

giantint from Richard Crandall, Perfectly Scientific, Inc., with special code for Fermat numbers.
GMP-ECM, based on the GMP multiprecision library.
mprime/Prime95 from George Woltman, is restricted to divisors of 2ⁿ-1 and to x86 architectures, but is faster than GMP-ECM for step 1.
Pari/GP and Magma also contain good ECM implementations (factorint(n,5) in Pari/GP).

Note: mprime/Prime95 and GMP-ECM use a common format, which makes it possible to perform step 1 with mprime/Prime95, and step 2 with GMP-ECM.

Other utilities

Tim Charron wrote a client/server utility for GMP-ECM.
20 May 2005: a new version (2.6.1) of the client/server utility, by Mark Rodenkirch.

ECM servers

Several ECM-factoring projects have set up a server that automatically distributes ECM tasks and collects found factors, using the client/server utility:

Paul Leyland's server is at 83.143.57.194, port 8194 [status], is loaded with Cunningham numbers, Cullen-Woodall numbers, and BMtR numbers;
The XYYXF project, whose aim is to factor numbers of the form x^y+y^x, has its server at euler.myip.org, port 34;
The ElevenSmooth project, searching factors of 2^3326400-1, has its server at wblipp.dynu.com, ports 8194 or 8195;
The Mersenneplustwo project, searching factors of Mersenne primes plus two, server at bearnol.is-a-geek.com, port 8194.